DIV. ii PHYSIOLOGY 317 



cell, which is the simplest form of reproduction, as well as in the 

 more complicated process of sexual reproduction. That the daughter 

 cells resemble the parent cells requires no explanation. The problem 

 of heredity appears when descendants are derived from the GERMS, 

 which are small portions of a complicated parent organism, by a 

 process of DEVELOPMENT. It is assumed that such germs possess 

 DETERMINANTS or GENES, which determine that an organism shall 

 react in a definite specific way to external factors. It appears 

 probable that these determinants are associated in the chromosomes of 

 the nucleus, but we know nothing as to the way in which they 

 influence the course of development. 



Such determinants must be present in the sexual cells of the 

 higher plants, ^and both in the male and the female cells. The 

 fertilised egg - cell must thus possess a double number of these 

 though a single organism is derived from it. That, originally at 

 least, the same determinants are present in all cells of the plant and 

 not only in the germ cells is shown by the phenomena of restitution. 



The problems of inheritance are of greatest interest in sexual 

 reproduction, in which the part played by the two parents in the 

 organisation of the progeny comes into prominence. These problems 

 can only be attacked by a consideration of hybrids, since the 

 individuals of a pure species have the same determinants. 



Hybrids ( 81 ). 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 repro- 

 duction. Occasionally, however, the sexual cells of different varieties, 

 species, 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. They are also spoken of 

 as HETEROZYGOTES or individuals derived from two dissimilar sexual 

 cells, in contrast to HOMOZYGOTES, which have arisen from the 

 union of sexual cells with identical determinants. Hybrids are as 

 a rule obtained more readily the closer the parent forms are to one 

 another, but this is not a rule without exceptions. 



Some families exhibit a tendency to hybridisation (Solanaceae, Caryophyllaceae, 

 Iridaceae, etc.) while in others hybrids are obtained with difficulty or not at all 

 (Papiliouaceae, Coniferae, Convolvulaceae, etc.). The behaviour of related genera 

 and species also is frequently very different. Thus species of Dianthus, Nicotiana, 

 Vefbascum, and Geum readily hybridise with one another, while those of Silene, 

 Solanum, Linaria, and Potentilla are difficult to hybridise. Hybridisation of 

 closely related species may frequently fail when more distant species can be 

 crossed. 



Hybrids also occur in nature, especially in the genera Salix, Rubus, Hieracium, 

 and Cirsium. That such natural hybrids do not occur oftener is due to the lack 

 of an opportune time or space for their development, and also to the Jact that in 

 the case of pollination of flowers with different kinds of pollen, that of their own 

 species seems as a rule more effectual in effecting fertilisation. 



