PHYSIOLOGY 309 



individuals themselves are HERMAPHRODITE and possess two kinds of 

 sexual organs, as in the case of the majority of Phanerogams. 



As, however, self-fertilisation takes place also in a number of 

 plants, either regularly or as a makeshift, it is evident that Avhat- 

 ever may be the advantage derived from a union of two distinct 

 individuals, it is no more essential for sexual reproduction than 

 for vegetative multiplication. Self-pollination, although regularly 

 occurring, frequently fails to occasion self-fertilisation, as often the 

 pollen will not develop pollen-tubes on the stigmas of the flower 

 (self-sterile) by which it was produced, but only on those of different 

 flowers (Secale cereale, Corydalis cava and some Crucif erae, Lobelia fulgens, 

 Verbascum nigrum, etc.) ( 116 ). 



The antipathy between the sexual organs of the same flower, in certain plants, 

 so greatly exceeds the bounds of indifference that they act upon each other as 

 poisons. Thus, for example, it is known of certain Orchids that pollination with 

 their own pollen causes the death of the flower, while in other cases the pollen is 

 killed in a short time by the stigmatic fluid of the same flower. 



In other instances, self-fertilisation occurs where cross-pollination 

 either is not effected, or else as an alternative to it (Wheat, Barley, 

 Canna, Viola species, Linum usitatissimum, etc.). By many plants, in 

 addition to the large " chasmogamous " flowers adapted to insect 

 pollination, small, inconspicuous flowers are produced which, usually 

 concealed underg'round or by the lower leaves, never open, and only 

 bear seeds which have been produced by self-fertilisation. In such 

 flowers the stamens no longer open, the pollen-tubes growing through 

 the wall of the anther to reach the stigma. In some plants the 

 majority of the seeds are derived from such CLEISTOGAMOUS flowers 

 (Viola), and sometimes their seeds alone are fruitful (Polycarpum 

 tetraphyllum possesses only cleistogamous flowers). As the greater 

 number of such plants, however, in addition to the seeds of the self- 

 fertilised small cleistogamous flowers, produce seeds resulting from 

 the cross-fertilisation effected in the larger flowers (Impatiens noli- 

 tangere, species of Lamium, Specularia perfoliata, Stellaria, Juncus 

 bufonius, etc.), the ancestral plants of the cleistogamous generations, 

 as well as their descendants, have, at least, the opportunity for 

 cross-fertilisation open to them. 



Special contrivances for ensuring the crossing of the sexual cells, 

 particularly by preventing self-pollination, are found to exist through- 

 out the whole vegetable kingdom. 



Self-pollination is most effectually avoided when the plants are 

 unisexual, that is when there are both male and female plants. 

 Such DIOECIOUS plants exist in almost all classes of plants from the 

 lower Cryptogams to the most highly developed Phanerogams (certain 

 Mucorineae, many of the lower Algae, species of Fucus, Marchantia, 

 Polytrichum, Equisetaceae, Taxus, Hemp, Hops, Date-Palm, etc.) In 



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