768 PART IV. — THE PHYSIOLOGY OF PLANTS. 



speaking, though the scion and the stock grow together so as to form one plant, 

 they do not affect each other, the scion retaining its own pecuHar characters 

 {e.g. grafting of fruit-trees, budding of roses). But it is affirmed that in some 

 cases the scion and stock do mutually effect each other, giving rise to shoots 

 which present a mixture of their respective characters : such are termed graft- 

 hybrids. 



An important fact connected with vegetative reprodaction is 

 that it is associated with a rejuvenescence of the protoplasm. For 

 example, when an adult cell of a unicellular plant, such as Pleuro- 

 coccus (Fig. 166, p. 236) divides, it gives rise, not to adult cells, 

 but to young ones : and a cutting produces a young plant, not an 

 old one. 



The relation of vegetative reproduction to the alternation of 

 generations is of importance. In the lower plants (e.g. Thallo- 

 phyta and Bryophyta) where the gametophyte is the conspicuous 

 generation, it is this generation which multiplies itself vegeta- 

 tively, although vegetative reproduction of a somewhat different 

 kind has been artificially induced in the sporophyte of some Mosses 

 (p. 362) ; but in the Phanerogamia it is exclusively the sporophyte 

 which thus multiplies itself. In the Pt^eridophyta, whilst vegeta- 

 tive multiplication of the sporophyte is common, the gametophyte 

 still retains this capacity in certain cases (some Ferns, p. 403; 

 Lycopodium, p. 425). Vegetative multiplication does not, as a 

 rule, affect the alternation of generations, each generation pro- 

 ducing its like : the exceptions are afforded by cases of apogamy 

 and apospory (see p. 87), in which the one generation is developed 

 vegetatively from the other ; that is, vegetative propagation 

 replaces spore-formation. 



§ 20. Spore-Reproduction (see p. 68). The highest degree 

 of reproductive capacity is that possessed by spores. Though 

 they are single cells, they are nevertheless capable, each by it- 

 self, of giving rise to a plant-body which, as in the higher plants, 

 may present complete morphological and histological differentia- 

 tion. 



The transition from vegetative propagation, through the unicel- 

 lular gemmae, to the simpler forms of spore-reproduction is so 

 gradual that it is difficult in many cases to distinguish them. 

 But there is one fact which may often serve as a criterion, and 

 that is that, at least in the lower plants, the spores are much more 

 resistent to unfavourable external conditions, such as drought and 

 extremes of temperature, than are gemmse. This is due partly 



