INFLUENCE OF THE ORIGIN OF THE CELLS ON FERTILISATION. 907 
physiologically dioecious. J. Scott states that Oncidium viicrochilum exhibits the 
same phenomena, the pollen not being potent on the stigma of the same flower 
while cross-pollination ensures fertilisation^; the pollen and stigma are therefore 
without function except to the stigma and pollen of a different flower. Similar phe- 
nomena have been described by Gärtner in the case oi Lobelia fulgens and Verbascum 
nigrum^ and in species of Begofua by Fritz Müller^. 
No less remarkable is another contrivance for the mutual fertilisation of diff"erent 
individuals of plants with hermaphrodite flowers, — Dimorphism'^ (or HeterostyHsm), 
consisting in a difference between different individuals of T;he same species with 
reference to their reproductive organs. In one individual the flowers all have a long 
style and short filaments, while in another individual all the flowers have a short style 
and long filaments, as in Linum perenne, Primula sinejisis, and other species of 
Primula. It sometimes happens also, as in Lythrum Salicaria and many species of 
Oxalis^, that the reproductive organs in the flowers of different specimens of the 
same' species exhibit three different relative lengths [Trimorphism), there being an 
intermediate length of style between the long-styled and the short-styled forms. In 
these cases of dimorphism and trimorphism Darwin and Hildebrand have shown that 
fertilisation is possible only (in the case of Linum perenne) or at least has the best 
result when the pollen of the long-styled flower is carried to the short-styled stigma 
of another plant, and vice versa ^. Where there are three different lengths of style, 
fertilisation succeeds best when the pollen is carried to the stigma which stands at the 
same height in another flower as the anthers from which the pollen came. It will be 
seen that this is but an expansion of the same rule. 
While in the very numerous diclinous, dichogamous, dimorphic, and trimorphic 
flowers, insects carry pollen from one flower to another, it is comparatively rare for 
cross-pollination to take place without the help of insects. This occurs in some 
Urticacese, as Pilea and Broussonetia, where the anthers emerge suddenly from the 
bud and scatter their light pollen in the air like a fine cloud of dust, which is then 
blown to the female organs of other flowers. In the Rye the arrangement is still 
simpler ; the flowers open separately, usually in the morning ; the filaments elongate 
rapidly and push the ripe anthers out of the pales; the anthers then hang down at 
the end of the long filaments, open, and aflow the heavy pollen to fall down, thus 
reaching the stigmas of other flowers lower down in the same spike or in neigh- 
bouring spikes, being assisted in this by the osciflations of the haulm under the 
influence of the wind ^. 
^ According to Fritz Müller (Bot. Zeit. 1868, p. 114), in some species oi Oncidumi the pollen- 
masses and stigmas of the same individual have a positively poisonous effect on one another. 
2 Fritz Müller, Bot. Zeit. 1864, p. 629. 
3 [Darwin, On the Two Forms, or Dimorphic Condition, in the Species of Primula, Journ. 
Proc, Linn. Soc. Bot. 1862, p. 77 ; ditto, On the Existence of Two Forms, &c. of the Genus Litmm, 
ibid,, 1863, p. 69; ditto, On Trimorphism in Lythrum Salicaria, ibid., 1S64, p. 169; ditto, On the 
Character and Hybrid-like Nature of the Offspring from the Illegitimate Unions of Dimorphic and 
Trimorphic Plants, Journ. Linn. Soc. 1868, p. 393; ditto, The Different Forms of Flowers on Plants 
of the same Species, 1877.] 
* Hildebrand, Bot. Zeit. 1871, Nos. 25, 26, 
^ [Darwin has given the name of legitimate to the union of two distinct forms, illegitimate to the 
fertilisation of long- or short-styled plants by pollen from flowers of their own form.] 
^ [For a detailed account of the very remarkable phenomena connected with the pollination of 
* 
