But absolutely conclusive evidence can be derived only from experiments, and by finding that pollen must be applied from the one form to the other in order to ensure complete fertility.
In order to show how much more fertile each form is when legitimately fertilised with pollen from the other form (or in the case of trimorphic species, with the proper pollen from one of the two other forms) than when illegitimately fertilised with its own-form pollen, I will append Table 6.33 giving a summary of the results in all the cases hitherto ascertained. The fertility of the unions may be judged by two standards, namely, by the proportion of flowers which, when fertilised in the two methods, yield capsules, and by the average number of seeds per capsule. When there is a dash in the left hand column opposite to the name of the species, the proportion of the flowers which yielded capsules was not recorded.
TABLE 6.33. Fertility of the legitimate unions taken together, compared with that of the illegitimate unions together. The fertility of the legitimate unions, as judged by both standards, is taken as 100.
Column 1: Name of species. Column 2: Illegitimate unions : proportional number of flowers which produced capsules. Column 3: Illegitimate unions : average number of seeds per capsule.
Primula veris : 69 : 65.
Primula elatior : 27 : 75.
Primula vulgaris : 60 : 54.
Primula Sinensis : 84 : 63.
Primula Sinensis (second trial) : 0 : 53.
Primula Sinensis (Hildebrand) : 100 : 42.
Primula auricula (Scott) : 80 : 15.
Primula Sikkimensis (Scott) : 95 : 31.
Primula cortusoides (Scott) : 74 : 66.
Primula involucrata (Scott) : 72 : 48.
Primula farinosa (Scott) : 71 : 44.
Average of the nine species of Primula : 88.4 : 69.
Hottonia palustris (H. Muller) : - : 61.
Linum grandiflorum (the difference probably is much greater) : - : 69.
Linum perenne : - : 20.
Linum perenne (Hildebrand) : 0 : 0.
Pulmonaria officinalis (German stock, Hildebrand) : 0 : 0.
Pulmonaria angustifolia : 35 : 32.
Mitchella repens : 20 : 47.
Borreria, Brazilian sp. : - : 0.
Polygonum fagopyrum : - : 46.
Lythrum salicaria : 33 : 46.
Oxalis Valdiviana (Hildebrand) : 2 : 34.
Oxalis Regnelli : 0 : 0.
Oxalis speciosa : 15 : 49.
The two or three forms of the same heterostyled species do not differ from one another in general habit or foliage, as sometimes, though rarely, happens with the two sexes of dioecious plants. Nor does the calyx differ, but the corolla sometimes differs slightly in shape, owing to the different position of the anthers. In Borreria the hairs within the tube of the corolla are differently situated in the long-styled and short-styled forms. In Pulmonaria there is a slight difference in the size of the corolla, and in Pontederia in its colour. In the reproductive organs the differences are much greater and more important. In the one form the stamens may be all of the same length, and in the other graduated in length, or alternately longer and shorter. The filaments may differ in colour and thickness, and are sometimes nearly thrice as long in the one form as in the other. They adhere also for very different proportional lengths to the corolla. The anthers sometimes differ much in size in the two forms. Owing to the rotation of the filaments, the anthers, when mature, dehisce towards the circumference of the flower in one form of Faramea, and towards the centre in the other form. The pollen-grains sometimes differ conspicuously in colour, and often to an extraordinary degree in diameter. They differ also somewhat in shape, and apparently in their contents, as they are unequally opaque. In the short-styled form of Faramea the pollen-grains are covered with sharp points, so as to cohere readily together or to an insect; whilst the smaller grains of the long-styled form are quite smooth.
With respect to the pistil, the style may be almost thrice as long in the one form as in the other. In Oxalis it sometimes differs in hairiness in the three forms.