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Volume 88, 1960-61
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– 34 –

Natural Populations

The table above shows the numbers of different sexes of plants counted in line transects at Cass in the season, summer 1957–58. The counts were made of every flowering plant on lines through each population. The ratios of the sexes one to another are also given. Included in the table also are figures for counts along the same transects at different parts of the flowering season and for some counts made of plants at Erewhon, in the Rangitata watershed in 1957, at the Macaulay River, and at Lake Tekapo in 1959. The plants counted at different parts of the flowering season will, in most cases, be different individuals, since any one plant flowers for a comparatively short period.

The above figures should be compared with figures for absolute numbers of fruit set and for percentage fruit set in the same species at Cass. The latter were arrived at by counting between 20–30 flowers in heads of individual plants, then marking these and finally counting the number of fruit set for each of these. For 20 plants (10 of each sex) about 25 flowers each were counted in an area of a few square.

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Numbers of Fruit Set by Counted Flowers;. P. “snow tussock”
☿ ☿
Plant No. of Flowers Counted No. of Fruit Set Plant No. of Flowers Counted No. of Fruit Set
1 22 1 1 23 14
2 24 3 2 28 9
3 22 0 3 25 11
4 29 0 4 25 2
5 21 0 5 27 12
6 24 1 6 23 18
7 23 0 7 33 26
8 23 0 8 23 2
9 22 0 9 24 3
10 22 0 10 24 5
Total 232 5 Total 255 102
– 35 –

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P. “short tussock”
☿ ☿
Plant No. of Flowers Counted No. of Fruit Set Plant No. of Flowers Counted No. of Fruit Set
1 25 0 1 24 15
2 24 1 2 25 11
3 24 0 3 25 23
4 24 1 4 24 23
5 22 1 5 21 19
6 24 0 6 27 20
7 24 3 7 22 16
8 24 2 8 23 20
9 27 1 9 25 24
10 24 0 10 23 17
Total 243 9 Total 239 188

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P. prostrata
☿ ☿
Plant No. of Flowers Counted No. of Fruit Set Plant No. of Flowers Counted No. of Fruit Set
1 23 0 1 24 5
2 27 0 2 22 0
3 24 0 3 23 19
4 22 0 4 21 11
5 22 1 5 23 10
6 24 0 6 23 4
7 21 0 7 22 16
8 24 2 8 23 7
9 24 3 9 23 5
10 23 4 10 25 19
Total 234 10 Total 229 96
– 36 –

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P. traversii
☿ ☿
Plant No. of Flowers Counted No. of Fruit Set Plant No. of Flowers Counted No. of Fruit Set
1 23 0 1 27 23
2 24 7 2 22 6
3 25 13 3 22 16
4 28 5 4 32 28
5 24 20 5 23 22
6 23 11 6 35 35
7 22 15 7 23 17
8 26 8 8 32 30
9 27 7 9 33 31
10 25 2 10
Total 247 88 Total 249 208

chains. The range of variation in fruit set from plant to plant both in females and hermaphrodites is of interest. Figures for P. prostrata, P. “snow tussock” and P. “short tussock” are fairly consistent, but those for P. traversii are more variable, as in seen in the table below.

It is possible that error could arise through loss of some fruit before counting P. “snow tussock” in a shady gully may have been affected in this way since a few female plants had far fewer fruits than the expected number. This may account for some of the variation in female fruit set. For this latter group of plants two sets of calculations, one involving all the plants and the other excluding those with very low fruit set, are given in the percentage fruit set table.

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Percentage Fruit Set in Plants of Different Sexes
Species Mean % Fruit Set Ratio
☿ ♀ ♀ ☿
P. “snow tussock” low figures included 40.0 2.15 16.7 : 1
P. “snow tussock” low figures excluded 56.25 2.15 26.1 : 1
P. “short tussock” 78.66 3.72 21.4 : 1
P. prostrata 41.92 4.27 9.8 : 1
P. traversii 83.54 40.64 2.0 : 1
– 37 –

The importance of these tables in showing that, firstly, the different Pimelea spp. vary considerably with respect to proportions of different sexes in the populations and secondly, population differences in the same species from area to area indicate that breeding system structure varies from population to population. The figures given by Godley (1955) for variation in proportion of females to hermaphrodites in Fuchsia from area to area show similar conditions. The differences from species to species are probably inherent in the genetics of the plants, but the population differences may be due to selective influences. An examination of these figures shows that at Cass both P. “snow tussock” and P. “short tussock” with 50% each of females and hermaphrodites in the population also have about 20 female to 1 hermaphrodite fruit set. This is important as a compensating factor, allowing the high proportion of females to be maintained in the field. On the other hand P. prostrata and P. traversii, with only about 30% of female plants in the population, have about 10 female to 1 hermaphrodite and 2 female to 1 hermaphrodite fruit set respectively. The higher proportion of hermaphrodite fruit set determines that fewer females may be retained in the population. It is to be expected that the Erewhon population of P. prostrata has yet a lower female fruit set. Low fruit set in hermaphrodites may perhaps be connected with incompatibility. Although these figures are indicative, and most useful in gauging the nature and extent of gynodioecy in Pimelea spp., the picture would be more complete with a study of the proportions of female to hermaphrodite seed which viable. This was not carried out owing to lack of time and difficulty of germinating seed.

The differences in fruit set compared with percentage numbers of the different sexes in the field in P. prostrata and P. traversii indicate differences in the sex determination mechanisms. It will be noticed that the percentage of female flowered plants counted in the populations of P. prostrata and P. traversii early in their flowering seasons was greater than at a later stage. There may be some connection in this with hormone balance. The higher proportion of female flowers open early in the season could, however, if insect vectors are active, contribute to the high proportion of females in the population. The competition between insects for flowers might have some bearing on this.

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