55] CHAP. IV. SPECIAL PHYSIOLOGY OF REPRODUCTION. 229 



which may be regarded as the highest of all, because it approaches 

 most nearly to spore-reproduction, and involves the entire develop- 

 ment of all the new members, is that of gemmae in which the 

 vegetative reproductive body is not merely a modified member of 

 the parent, but is a special development consisting in some cases 

 of only a single cell (e.g. gemmae of some Algae and Liverworts ; 

 oidium-cells of Fungi). Something of the same kind occurs 

 amongst the higher plants, such as some Ferns, Bryophyllum, etc., 

 where an entirely new structure, a bud, is developed on the leaf, 

 and produces stem, leaves and roots ; it is in this way that Bego- 

 nias are artificially propagated (see p. 136). 



An interesting artificial mode of vegetative propagation is that known 

 as grafting or budding, in which a young shoot or a bud, termed the 

 scion, of one plant is inserted into the stem of another, though allied 

 plant, the stock (see p. 156) , the scion and the stock grow together so as to 

 form one plant, the scion retaining its own peculiar characters (e.g. graft- 

 ing of fruit-trees, budding of roses). 



An important fact connected with vegetative reproduction 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. 137), 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. Bryophyta) 

 where the gametophyte is the conspicuous generation, it is 

 this generation which multiplies itself vegetatively, although 

 vegetative reproduction of a somewhat different kind has been 

 artificially induced in the sporophyte of some Mosses ; but in the 

 Phanerogamia it is exclusively the sporophyte which thus multi- 

 plies itself. In the Pteridophyta, whilst vegetative multiplication 

 of the sporophyte is common, the gametophyte still retains this 

 capacity in certain cases (some Ferns ; Lycopodium). Vegetative 

 multiplication does not, as a rule, affect the alternation of genera- 

 tions, each generation producing its like : the exceptions are 

 afforded by cases in which the one generation is developed vegeta- 

 tively from the other; that is, where vegetative propagation 

 replaces spore-formation. For instance, in some Ferns, the pro- 

 thallium is developed vegetatively from the Fern-plant, without 

 the intervention of spores (apospory) ; and the Fern-plant vegeta- 

 tively from the prothallium, without the intervention of sexual 

 organs (apogamy}. 



