766 PART IV. THE PHYSIOLOGY OF PLANTS. 



the leaf is eventually raised to its normal position by the gradual 

 resumption of turgidity, that is, of the contracted condition of the 

 protoplasm, by those cells of the pulvinus which had become 

 flaccid as the result of the assumption of the relaxed state on 

 stimulation. In a word, the contracted state is that in which 

 alone motile protoplasm is capable of doing mechanical work. 



CHAPTER IV. 

 SPECIAL PHYSIOLOGY OF EEPKODUCTION. 



18. Introductory. It has been already stated (see pp. 3, 

 67, 670) that reproduction consists essentially in the throwing 

 off by the individual of a portion of its protoplasm which does 

 not merely grow but developes into a new organism ; and that two 

 modes of reproduction, vegetative multiplication and spore-reproduc- 

 tion may be conveniently distinguished, though they are not 

 absolutely distinct. 



Reproduction has been considered so far mainly from the 

 morphological standpoint, and it now remains to discuss it from 

 the physiological point of view. The most important general 

 consideration is that reproduction is a function of embryonic, as 

 distinguished from adult, protoplasm. But it must not be over- 

 looked that all embryonic protoplasm is not necessarily reproduc- 

 tive : and it is interesting to trace the differences in this respect, 

 presented by various kinds of embryonic protoplasm. To begin 

 with, there is no doubt that the merismatic cells of the cambium 

 are embryonic (see p. 18) ; but they are not at all reproductive, for 

 they cannot give rise to a new member, still less to a new organ- 

 ism ; they can only add to the bulk of the body of which they form 

 part, by the development of new tissue. Again, the protoplasm 

 of a growing-point is embryonic, but it is only imperfectly repro- 

 ductive ; it possesses this property to the extent that it not only 

 contributes to the increase of the member to which it belongs, 

 but also developes new members. Finally, the protoplasm of a 

 reproductive cell, such as a spore, is embryonic and is completely 

 reproductive; for it does not in any degree contribute to the bulk 

 of the parent-organism, but gives rise to a new individual. 



Whilst the embryonic character of cytoplasm is probably 

 dependent upon the presence of a large proportion of kinoplasm 

 (see p. 95), there is at present no means of distinguishing by 



