SECT. II 



PHYSIOLOGY 28J 



arise iu vegetative multiplication, as may happen, according to Beyerinck, even in 

 Bacteria which reproduce only by division into two {^^). 



In contrast to the vegetative progeny the sexually produced 

 descendants, as a rule, cannot completely resemble the mother plant, 

 but must combine the characters of both parents. The more these 

 differ from each other, the more striking will be the visible effect of 

 fertilisation. Since, however, the individuals of a species frequently 

 differ but slightly to our observation, the effect of the mingling of 

 characters may be hardly noticeable in a typical case of fertilisation. 

 Fortunately it is often possible to combine sexually individuals of 

 distinct races, varieties, species and even genera, and the progeny 

 resulting from such hybrid unions exhibits in the clearest way the 

 mixture of characters of the two organisms (see below). 



The fact already mentioned that many arrangements in the flowers of the higher 

 plants lead to a crossing being effected supports this assumption of the significance 

 of fertilisation. While it is quite possible that the egg- and sperm-cells of the 

 same flower may differ from one another in their hereditary properties, it is 

 probable that the sexual cells of distinct flowers will exhibit larger difl'erences. It 

 is of great interest that plants exist in which self-pollination does not result in the 

 development of seeds since in extreme cases the pollen is incapable of growth on 

 the stigma of the same flower (self-sterile flowers). Thus, for example, the flowers 

 of CorydaUs cava and Sccale cormdum are self-sterile. In certain orchids this 

 goes so far as to result in injurious effects of the pollen on the stigma, or the stigma 

 on the pollen when cross-pollination has not taken place. In other cases {e.g. in 

 some Papilionaceae) the failure to set seed after self-pollination depends on the 

 pollen being able to grow only on a stigma which has been rubbed. While this is 

 not a case of actual self-sterility, the adaptations of the flower are such that the 

 growth of the pollen will usually take place only after the visit of an insect ; a 

 visiting insect will, however, as a rule bring pollen from another flower. 



VI. Hybrids 



The union of two sexual cells is, as a rule, only possible when 

 they are derived from individuals of the same species ; it is only then 

 that they fuse together in the act of sexual reproduction. Occasion- 

 ally, however, the sexual cells of difterent varieties, sjjecies, or even 

 genera have been shown to be able to unite and produce descendants 

 capable of development. Such a union is termed hybridisation, and 

 its products HYBRIDS. 



By the demonstration of the possibility of hybridisation, the 

 sexuality of plants, for a long time doubted, was indisj)utably proven. 

 With this object in view, and following the work of Fairchild, in 

 1717, hybrids were raised in great numbers by Kolreuter as early 

 as 1761 (^^). It also demonstrated that the real purpose of sexual 

 union was the combination of the properties of both parents, for 

 transitional forms are found among hybrids which in many character- 

 istics resemble the male and in others the female ancestor (hetero- 



