C. The Growth-forms and their Relation more especially to the Vertical Distribution.
On account of the constantly high humidity in the lowland forests of Westland, the Hymenophyllaceae are there to be found at their optimum development. Throughout Westland the epiphytic station is, taking the family as a whole, the characteristic one, and a number of the species adopt a luxuriant pendulous habit with irregularly elongated fronds. This very luxuriance serves to throw into greater prominence two facts—viz., that certain thorough-going epiphytes are able to adopt a stunted and imbricated frond-form with a mat-like habit of growth, and in this state to flourish even in the forest-canopy and in other exposed stations, other epiphytic species being quite incapable of doing this; and, secondly, that certain terrestrial or low epiphytic species preserve the deltoid form of frond unmodified. The growth-form of each species and the extent to which it can be modified must thus be regarded as an important factor in the determination of its distribution. In the case of some of the species the growth-form will restrict the distribution, but with others the ability to modify it will act in the opposite direction.
(a.) The Tufted Growth-form.—This growth-form is known to be confined almost exclusively to certain species of the genus Trichomanes. The tufted species, T. elongatum, T. strictum, and H. pulcherrimum, do not occupy any considerable place in the physiognomy of their surroundings, since they cannot form sheets as do the creeping species. They occur generally singly, although a close search will often reveal the presence of a scattered colony which has originated from a single old plant. The two first-named species prefer secluded overhung situations where frequently no dripping water can reach the fronds. In the case of T. elongatum, whose frond-lamina is more extensive than that of T. strictum, the presence of epiphyllous mosses and hepatics, however, proves the constant wetness of the fronds, and shows that in its secluded station there must be considerable condensation of water-vapour upon the frond-surface. The root-system of both species is well developed and the stem-xylem is large, so that, when also the small number of fronds is considered, it must be concluded that these plants obtain most of their water, and certainly all their nutrient salts; by root absorption. In his laboratory experiments with Jamaican species Forrest Shreve (26, p. 194) found that in the case of T. rigidum, whose growth-form is identical with that of the two species here considered, the extremely low water-loss from surface-dry leaves in a very moist atmosphere could not be met by root absorption. At the same time, he states that no other Jamaican species, even the most hygrophilous, behaved in this way, and he is inclined to find the explanation of it in T. rigidum in the fact that the lateral walls of the cells of its frond-lamina are greatly thickened and that thereby the rapid passage of water outwards from the veins is hindered. These tufted terrestrially-growing species of Trichomanes show no marked modification in the frond-form in the way of increasing the frond absorption, beyond the facts, previously mentioned, that there is a very slight zone of multi-layered lamina in the frond of T. elongatum where the larger veins converge, and also that the frond-lamina of this species is more extensive than that of T. strictum, so that in this respect they may be regarded as representing more or less nearly the original frond-form of the family. At the same time, it is, of course, possible that in other ways they may have departed from the primitive character of the family in accordance with their specially hygrophilous habits. Their growth-form restricts
them to situations where they can root in the soil and where this is continuously damp.
H. pulcherrimum is a tufted species which in Westland occurs altogether as an epiphyte. Its fronds are large, with a much greater total extent of lamina than in the two last-named species. Moreover, the frond-form is not fixed, as in the two others, but is readily modified into a pendulous, much-elongated form. Pendulous fronds are frequently bathed in rainwater which has splashed from the leaves of the trees or has run down over the surface of the bark and through the mass of moss and humus in which the plant is growing, so that this species is not so dependent upon root absorption as the two above mentioned. Its root-system is, however, well developed, and it is confined to forks of trees and other epiphytic places where there is a considerable accumulation of humus. It has been indicated above in the paragraph dealing with Otira that this species probably, after all, demands a constant root supply of water rather than a constantly high atmospheric humidity, although, of course, it will attain its most marked luxuriance if the latter factor be also present. It would appear that H. pulcherrimum cannot occur on the flat floor of the forest as does T. strictum, for it has departed from the typically erect terrestrial habit; nor can it descend to the lowlands, for there the branches of the trees do not, as a rule, carry much moss and humus, and are liable to occasional drying.
(b.) The Open-creeping Terrestrial Growth-form.—The representatives of this class are H. demissum and H. bivalve. The fronds of these species are strongly erect or somewhat decurved, and are of an invariable deltoid form, and hence of limited size. The rhizome and root-system is well developed. Among the Hymenophyllaceae the typical epiphytic species show special frond-modifications in accordance with this station in the way of the extension of the lamina, and especially in the departure from the erect deltoid form of frond of limited size; but such modifications are altogether lacking in H. demissum and H. bivalve, even when they become low epiphytes, as the latter frequently does, and the former occasionally. It may well be that the inability to modify the frond-form in order to extend its function of absorption prevents these two species from becoming thorough-going epiphytes. Of the two, H. demissum possesses the greater extension of lamina, so that it is not obvious why this species should show the much less marked tendency to become epiphytic. The adoption of the creeping habit certainly makes it possible for a plant to take up an epiphytic station.
(c.) The Open-creeping Epiphytic Growth-form. — The small epiphytic species are considered by themselves below. In the species of this third class there is usually a modification of the frond in accordance with the epiphytic habit. H. australe is a good example of the transition from the terrestrial to the epiphytic station, occurring in Westland always at the bases of large trees and climbing upwards a few feet. It retains the erect deltoid form of frond, but shows a considerable extension of the lamina by means of the broad crisped wings. It occasionally is pendulous, and the fronds then show a marked increase in length. H. atrovirens, which is probably a form of this species, is erect with deltoid or lanceolate fronds when terrestrial, but pendulous and elongated when growing on vertical rock-faces in sheltered ravines. The rhizome and stipe of T. reniforme is comparatively stout, but, with the exception of H. Malingii, the frond shows the greatest degree of modification in the New Zealand family. The
study of the sporelings of this species shows that the completely webbed character of the mature frond, and possibly also the multi-layered condition of its lamina, are to be regarded as special modifications. Another stoutly-growing species with considerable frond-modification is H. dilatatum. Here the extent of lamina is certainly, taking into consideration the size of the frond, the greatest in any of the New Zealand species. H. scabrum, H. sanguinolentum, and H. multifidum also show a large frond-development in the pendulous state, the first-named attaining an extreme length of 2 ½ ft. to 3 ft. In the mid-epiphytic station in the lowland forests the lateral pinnae of the fronds of H. sanguinolentum and of H. multifidum, as well as the main rhachis, frequently tend to elongate into tails. H. villosum, although a characteristic epiphyte, shows a poor extent of frond-lamina, and the frond preserves more or less invariably the erect deltoid form. To this statement must be added the fact that it is a mountain and not a lowland plant, and that whereas the mid-epiphytic station in the lowland forests enables the species to make the most use of atmospheric water by frond absorption, the same station in the mountain-forests, especially at such elevations as H. villosum can ascend to, where the atmospheric humidity is more variable, is of value rather on account of the water-supply in the damp mossy clothing of the tree-branches. When, however, it occurs as a low epiphyte at the bases of the ranges, in which localities and station there is a more constantly high atmospheric humidity, its fronds frequently show a well-marked tendency to an elongation of the main rhachis, and they may also be pendulous.
The question arises regarding the open-creeping, strong-stemmed epiphytes such as H. dilatatum, H. scabrum, and T. reniforme, which in creek-beds at the base of the ranges can occur rupestrally and adopt a more erect habit of growth, as they do also very generally in the more light forests of other parts of New Zealand—why do they not also occur on the floor or on fallen logs in the lowland forests of Westland? It may be that with such species the need for light in the heavy dense forests causes them to keep to the epiphytic station, and it is to be noted that where in Westland they do occur terrestrially at higher altitudes it is in such less-shaded localities as creek-beds.
As has been mentioned, the fronds of the pendulous epiphytes will frequently be bathed in water which has soaked down from the branch or trunk on which they are growing. No doubt the fronds also absorb nutrient salts in this way. In his experiments already quoted, Forrest Shreve (26) showed that the ground-growing, more hygrophilous Jamaican species do not absorb uncondensed atmospheric moisture by the fronds, but that the high epiphytes do, although the actual amount so obtained is small. The rhizomes and roots of the large-growing mid-epiphytic New Zealand species are not usually embedded in moss, but simply cling closely to the tree-surface, so that there is no considerable root water-supply available for them. T. reniforme, whose fronds are generally rigidly erect, prefers the larger surfaces of tree-trunks, but the other species of this class, with the exception of H. australe and H. villosum, prefer the under-side of horizontal boughs from which they can hang free.
(d.) The Thin-stemmed Close-creeping Epiphytic Growth-form.—There are a few species which show in the filiform nature of their rhizomes and stipes, and, in the case of certain species, in their hairy character, a still further modification in accord with the epiphytic habit. H. flabellatum, H. rarum, and H. Malingii are typical members of this class, and also certain of the smaller-growing species. They are always pendulous, the first two being
characteristically confined to the under-side of inclined trunks and large boughs, and they grow generally in close colonies. It is obvious that the thread-like stems of these species could not carry an adequate water-supply from the substratum even if this were available. On the other hand, there is a very large extent of lamina in proportion to the size of the frond, this being especially the case in H. rarum; while in H. Malingii, as has been noted, there is a very remarkable and effective frond-modification for the purpose of frond absorption. H. ferrugineum is also invariably pendulous with overlapping fronds in its station on tree-fern stems, and the rhizome and stipe is slender, though not so much so as in the three above-mentioned species.
It is significant that these four species, which are thus so dependent upon frond absorption, show withering in times of drought to a markedly less degree than do other epiphytic species. In the case of H. Malingii the nature of the frond-tissues readily explains this. It has been suggested that the peculiar colour of the fronds of H. rarum and H. flabellatum may possibly serve to mask the cell-protoplasm from excessive transpiration. H. ferrugineum is one of the hairy species. Its fronds are almost rusty-brown in colour owing to the dense clothing of stellate hairs on the margins of the segments and on the costae on both surfaces of the frond (see Plate 76). So closely do the hairs interlock that they cannot help but shield the lamina from air-movements. However, it must be remembered that this species is more restricted in its distribution than the other three, and is distinctly hygrophilous. H. ciliatum, which very doubtfully occurs in New Zealand, seems to be of much the same slender pendulous growth-form as H. ferrugineum. In this species also the margins of the segments, and to a less extent the costae, are clothed with stalked stellate hairs. Forrest Shreve notes (26, p. 193) that the three Jamaican hairy species of Hymeno-phyllum are the most resistant to drying and insolation of any. From experiments he concludes that the cavities of the frond-hairs have no great importance as reservoirs of water. He observes also that under natural conditions these hairy fronds do not often become saturated with rainwater, a fact which I may state is true also of the New Zealand hairy species. However, it may well be that the hairs hinder transpiration, and this will certainly be so in the case of the densely-clothed young fronds. I may add that thick-walled colourless unicellular bristles are borne in large numbers on the margins of the prothalli of the terrestrial T. elongatum, and are especially in evidence overlapping their growing-points. Each arises from a single otherwise unaltered marginal cell. Whether the hairs in this case serve as a mechanical strengthening or for holding a film of water cannot here be stated. H. flabellatum is characterized by a dense woolly tomentum on the rhizome and stipe of the mature frond, and this on the young frond shields the lamina also. This will act as a protection for the growing parts of the plant, and hence may be considered of special importance with respect to the epiphytic station. In this connection it is to be noted that on the mountain-flanks this species may frequently be found showing the tomentum as a persistent although more or less scanty clothing on the rhachis and costae of the mature as well as of the younger fronds.
(e.) The Mat Growth-form of the Small Species.—The nine smallest species —viz., T. Colensoi, T. humile, T. Lyallii, T. venosum, H. rufescens, H. Tunbridgense, H. peltatum, H. minimum, and H. Armstrongii—are all creeping mat-formers, a growth-form which is, of course, very effective both in hindering transpiration through the overlapping of the fronds and also in holding water and humus against the roots. The stipe of T. humile and
T. venosum is flexible as well as slender, so that their fronds are pendulous and irregularly elongated. In this respect these two species are more modified than the others of this class. Occasionally, however, erect fronds of T. venosum may be found which are less modified from the deltoid form (see Plate 72). In especially humid localities the fronds of H. peltatum and H. Tunbridgense also may become pendulous and elongated. The tiny fronds of the other small species preserve a more or less rigid habit of growth, even when, as in T. Colensoi, T. Lyallii, and H. rufescens, they keep to overhung stations, and the fronds remain rigid and practically unmodified in form. Most of these small species are probably best interpreted as reduction forms. The frond of H. rufescens is typically deltoid, and it is to be noted that H. flabellatum, to which the other species is undoubtedly closely allied, also frequently adopts a very similar form in mountain stations.
The hairy covering of the fronds of H. rufescens and T. Lyallii probably acts as a protection against drying in the same way as has been suggested for H. ferrugineum, and thus plays a part in enabling these species to endure a somewhat more exposed position than such other mountain species as T. Colensoi. No doubt the base of large trees at the misty altitudes at which these two species abound is for the most part a sheltered and humid station; but the reason why they prefer high to low altitudes is best sought in the supposition that they are mountain forms of lowland plants.
In the case of all these small delicate species the mat is the natural growth-form when growing on surfaces of any extent, although H. Armstrongii and H. minimum are small enough to be able to bury their fronds in moss, and thus to spread as more open creepers on to the smaller surfaces of tree-branches. H. peltatum alone of these small species can become stunted with imbricating pinnae, and it is in this form that it occurs in exposed positions. It is certain of these small species which show, especially in their more exposed stations, the mat-form in the most pronounced state in which it is to be found in the family. Possibly the inability to shield the lamina by imbrication and inrolling of the pinnae is an important reason why such small species as H. Tunbridgense and the other typically low epiphytes of this class are so restricted in their vertical distribution.
(f.) The High Epiphytic Stunted and Imbricated Mat Growth-form.— The last growth-form to be considered is that illustrated by certain normally large-fronded species, which, on account of their ability to adopt the mat habit together with an extremely stunted and imbricated form of frond, accompanied in certain of the species by inrolling of the frond-segments, are able in the lowlands to endure high epiphytic conditions, and, in the case of two of the species, to exist in exposed subalpine stations. These are H. sanguinolentum, H. multifidum, H. villosum, H. rarum, and H. flabellatum. These species all occur commonly along with the small H. Armstrongii in the tops of the forest-trees in the lowlands, both in the heavy forest of the coastal flats and of the terraces, and also in the black-pine and white-pine stands of the river-flats, the latter of these localities being more exposed to the mountain winds. As has been noted, H. villosum does not occur below the middle lowlands.
H. villosum and H. multifidum are able to endure greater degrees of desiccation than the other species, as is shown by the fact that they alone flourish in exposed positions above the forest-line. As high epiphytes in the lowlands these two species and H. sanguinolentum show the stunting of the frond and imbrication of the pinnae with inrolling of the segments,
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T. reniforme Forst. × 7/9.
but they do not here adopt so characteristically the mat growth-form. On the other hand, the frond-segments of H. rarum and of H. flabellatum are apparently unable to inroll, but these two species keep their high epiphytic station by the imbrication of the pinnae along with the adoption of the close mat growth-form. For this reason they keep to the larger surfaces of the main trunk and of the larger limbs in the tree-tops in the lowlands, whereas the other three species creep out also on to the small branches. It must be noted also that as a high epiphyte H. multifidum, although much stunted, does not adopt the peculiar “mountain form,” this being only found in its terrestrial station.
Above the forest-line, and in exposed subalpine localities generally, H. villosum and H. multifidum adopt the close mat growth-form. Of these two species the former can certainly exist in the more exposed positions, H. multifidum seeming to need always a damper, more shaded situation. Moreover, whereas the latter is here always rupestral, the former is frequently a low epiphyte, being then an open creeper in the moss. Possibly it is the possession of the villous clothing of the fronds and rhizomes which gives H. villosum the extra advantage, although it must be said that at these altitudes H. multifidum also shows this character, but to a much less extent. As a mountain plant the fronds of the last-named species are typically deltoid in form and in the most exposed situations are strongly decurved so as almost to bury themselves in the mossy substratum.
As indicated previously, the significance of the imbrication of the pinnae, the inrolling of the frond-segments, and the stunting and overlapping of the fronds of these mat-formers lies in the fact that transpiration is thereby hindered, and probably also water is held against the frond-surface. More-over, the underlying stratum of interwoven rhizomes and roots, and the accumulated humus of the dead fronds and of the moss, is able to hold water over considerable periods of drought.