CROSS-POLLIXATIOX— HYBRIDISATION 521 



This arrangement is generally spoken of as dimor2)hism, of 

 which, however, it is only one form. Lijthrum Salicaria is 

 trimorphic, having two sets of stamens of different lengths and 

 a style which differs from both. There are three modes of 

 arrangement of these organs, and, as in the Primrose, the most 

 serviceable pollination is that which takes place when pollen 

 from a stamen of a certain length is applied to a stigma of the 

 same length. 



Other arrangements are physiological rather than structural. 

 Of these the strangest is what is caMedj^rej^otencij. When a stigma 

 of a flower exhibiting this property is pollinated by pollen from 

 its own stamens and at the same time by pollen taken from 

 another flower, the latter is always the originator of the gamete 

 by which fertilisation is effected. Some flowers show self- 

 sterility, that is, they are never fertilised if only pollinated by 

 pollen from their own stamens ; in some few cases their own 

 pollen acts as a poison to them. 



Though cross-pollination is thus seen to be most advantageous, 

 it is not universal. Self-pollination occurs in many plants ; 

 indeed, in some, special means have been developed to secure 

 it in case cross-pollination fails to be effected. Only one of 

 these need here be alluded to ; this is cJeistogamy, or the pro- 

 duction of special flowers which do not open, in addition to the 

 normal ones. The pollen grains put out their pollen tubes in 

 the unopened flowers, sometimes not even escaping from the 

 sporangia. The tubes grow towards the stigmas, and, penetra- 

 ting them, reach the ovules as in the case of the normal flower, 

 fertilisation resultmg in the same way. 



Though cross-fertilisation is seen to be most advantageous, it 

 is only possible within certain limits. For a new individual to 

 be produced, the sexual cells taking part in the process must 

 have a certain degree of relationship ; thus the antherozoid of a 

 moss cannot fertilise the oosphere of a fern. The most favourable 

 degree of relationship is that the two gametes shall be produced 

 by different plants of the same species. Such a union results in 

 greater numbers and greater vigour in the offspring. Plants, 

 however, not so closely related, may produce offspring ; thus we 

 may have the union of gametes of plants standing to each other 

 in the relation of varieties of the same species, or very frequently 

 of distinct species belonging to the same genus, or even of 

 species of different genera. Sucli fertilisation is known as 

 hybridisation. 



Hybrids generally exhibit peculiarities of form and structure 



