sect, ii PHYSIOLOGY 277 



in sexual reproduction to ensure that the uniting cells have been 

 developed from different individuals of the same species, an equal- 

 ising influence is exerted which tends to maintain the permanence 

 of the species as a whole. Any accidental variations in the form 

 or properties of one individual of a species would, through crossing 

 with others normally developed, disappear in the descendants, while 

 the descendants by vegetative reproduction would retain them. A 

 phenomenon of not infrequent occurrence, and one which shows, on 

 the other hand, the persistency with which inherited attributes are 

 retained in sexual reproduction, is the unexpected reappearance in the 

 descendants of the attributes of former generations (atavism). 



While, on the one hand, sexual reproduction tends to maintain the 

 unchangeability of the form by abolishing isolated variations, on the 

 other hand, variations may be confirmed in the descendants when they 

 were similarly manifested by both parents. As a result of the union 

 between individuals of different varieties, or species, or even of differ- 

 ent genera (cf. Hybridisation, p. 289), offspring may be produced 

 which, if not sterile, have a remarkable tendency to variation and 

 so to the formation of new forms. 



It is in this influence exerted upon the quality that the chief 

 difference between sexual and vegetative reproduction is shown. By 



VEGETATIVE REPRODUCTION THE QUANTITATIVE MULTIPLICATION OF 

 THE INDIVIDUAL IS SECURED, WHILE BY SEXUAL REPRODUCTION A 



qualitative influence is exerted, which is of the greatest import- 

 ance for the continued existence of the species. Sexual reproduction 

 might therefore be spoken of as the qualitative reproduction of 

 the species, and vegetative reproduction as the quantitative repro- 

 duction of the individual. The vegetatively produced progeny consist 

 of unmixed descendants ; the sexually produced offspring, on the other 

 hand, are the result of a blending of the parents. 



Vegetative Reproduction 



Vegetative reproduction, the purely quantitative character of 

 which as a mere process of multiplication has been emphasised, exists 

 generally throughout the vegetable kingdom, and but few plants, some 

 of the Conifers and Palms, are altogether devoid of it. Mention has 

 already been made in considering artificial propagation (p. 228) that, 

 from the separate parts or single cells, or even from the naked 

 energides (Siphoneae) of many plants, the regeneration of a new and 

 perfect individual may ensue. In vegetative reproduction the process 

 is similar except that the separation of the part from the parent plant 

 is an organic one, occurring in the natural course of development. 

 The vegetative form of reproduction is manifested in various aspects, 

 and may be distinguished as a multiplication by means of multicellular 

 vegetative bodies (budding), or by single cells (spore-formation). 



