Part 4 (2/2)

Primula Sikkimensis : 57.

Primula cortusoides : 93.

Primula involucrata : 74.

Primula farinosa : 63.

We thus see that, with the exception of P. auricula, the long-styled flowers of all nine species are more fertile than the short-styled flowers, when both forms are illegitimately fertilised. Whether P. auricula really differs from the other species in this respect I can form no opinion, as the result may have been accidental. The degree of self-fertility of a plant depends on two elements, namely, on the stigma receiving its own pollen and on its more or less efficient action when placed there. Now as the anthers of the short-styled flowers of several species of Primula stand directly above the stigma, their pollen is more likely to fall on it, or to be carried down to it by insects, than in the case of the long-styled form. It appears probable, therefore, at first sight, that the lessened capacity of the short-styled flowers to be fertilised with their own pollen, is a special adaptation for counteracting their greater liability to receive their own pollen, and thus for checking self-fertilisation. But from facts with respect to other species hereafter to be given, this view can hardly be admitted. In accordance with the above liability, when some of the species of Primula were allowed to fertilise themselves spontaneously under a net, all insects being excluded, except such minute ones as Thrips, the short-styled flowers, notwithstanding their greater innate self-sterility, yielded more seed than did the long-styled. None of the species, however, when insects were excluded, made a near approach to full fertility. But the long-styled form of P.

Sinensis gave, under these circ.u.mstances, a considerable number of seeds, as the corolla in falling off drags the anthers, which are seated low down in the tube, over the stigma, and thus leaves plenty of pollen on it.

h.o.m.oSTYLED SPECIES OF PRIMULA.

It has now been shown that nine of the species in this genus exist under two forms, which differ not only in structure but in function. Besides these Mr.

Scott enumerates 27 other species which are heterostyled (1/13. H. Muller has given in 'Nature' December 10, 1874 page 110, a drawing of one of these species, viz. The alpine P. villosa, and shows that it is fertilised exclusively by Lepidoptera.); and to these probably others will be hereafter added.

Nevertheless, some species are h.o.m.ostyled; that is, they exist only under a single form; but much caution is necessary on this head, as several species when cultivated are apt to become equal-styled. Mr. Scott believes that P. Scotica, verticillata, a variety of Sibirica, elata, mollis, and longiflora, are truly h.o.m.ostyled; and to these may be added, according to Axell, P. stricta. (1/14.

Koch was aware that this species was h.o.m.ostyled: see ”Trevira.n.u.s uber Dichogamie nach Sprengel und Darwin” 'Botanische Zeitung' January 2, 1863 page 4.) Mr.

Scott experimented on P. Scotica, mollis, and verticillata, and found that their flowers yielded an abundance of seeds when fertilised with their own pollen.

This shows that they are not heterostyled in function. P. Scotica is, however, only moderately fertile when insects are excluded, but this depends merely on the coherent pollen not readily falling on the stigma without their aid. Mr.

Scott also found that the capsules of P. verticillata contained rather more seed when the flowers were fertilised with pollen from a distinct plant than when with their own pollen; and from this fact he infers that they are sub- heterostyled in function, though not in structure. But there is no evidence that two sets of individuals exist, which differ slightly in function and are adapted for reciprocal fertilisation; and this is the essence of heterostylism. The mere fact of a plant being more fertile with pollen from a distinct individual than with its own pollen, is common to very many species, as I have shown in my work 'On the Effects of Cross and Self-fertilisation.'

Hottonia pal.u.s.tris.

This aquatic member of the Primulaceae is conspicuously heterostyled, as the pistil of the long-styled form projects far out of the flower, the stamens being enclosed within the tube; whilst the stamens of the short-styled flower project far outwards, the pistil being enclosed. This difference between the two forms has attracted the attention of various botanists, and that of Sprengel, in 1793, who, with his usual sagacity, adds that he does not believe the existence of the two forms to be accidental, though he cannot explain their purpose. (1/15. 'Das entdeckte Geheimniss der Nature' page 103.) The pistil of the long-styled form is more than twice as long as that of the short-styled, with the stigma rather smaller, though rougher. H. Muller gives figures of the stigmatic papillae of the two forms, and those of the long-styled are seen to be more than double the length, and much thicker than the papillae of the short-styled form. (1/16. 'Die Befruchtung' etc. page 350.) The anthers in the one form do not stand exactly on a level with the stigma in the other form; for the distance between the organs is greater in the short-styled than in the long-styled flowers in the proportion of 100 to 71. In dried specimens soaked in water the anthers of the short-styled form are larger than those of the long-styled, in the ratio of 100 to 83. The pollen-grains, also, from the short-styled flowers are conspicuously larger than those from the long-styled; the ratio between the diameters of the moistened grains being as 100 to 64, according to my measurements, but according to the measurements of H. Muller as 100 to 61; and his are probably the more accurate of the two. The contents of the larger pollen-grains appear more coa.r.s.ely granular and of a browner tint, than those in the smaller grains. The two forms of Hottonia thus agree closely in most respects with those of the heterostyled species of Primula. The flowers of Hottonia are cross-fertilised, according to Muller, chiefly by Diptera.

Mr. Scott made a few trials on a short-styled plant, and found that the legitimate unions were in all ways more fertile than the illegitimate (1/17.

'Journal of the Linnean Society Botany' volume 8 1864 page 79.); but since the publication of his paper H. Muller has made much fuller experiments, and I give his results in Table 1.13, drawn up in accordance with my usual plan:--

TABLE 1.13. Hottonia pal.u.s.tris (from H. Muller).

Column 1: Nature of the Union.

Column 2: Number of Capsules examined.

Column 3: Average Number of Seeds per Capsule.

Long-styled by pollen of short-styled. Legitimate union: 34 : 91.4.

Long-styled by own-form pollen, from a distinct plant. Illegitimate union: 18 : 77.5.

Short-styled by pollen of long-styled. Legitimate union: 30 : 66.2.

Short-styled by own-form pollen, from a distinct plant. Illegitimate union: 19 : 18.7.

SUMMARY:

The two legitimate unions together: 64 : 78.8.

<script>