2l6 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



and that consequently these plants are specially adapted for 

 propagation by leaves. How is it then that all plants cannot 

 be reproduced in this way ? No one has ever grown a tree 

 from the leaf of the lime or oak, or a flowering plant from the 

 leaf of the tulip or convolvulus. It is insufficient to reply that, in 

 the last-mentioned cases, the leaves are more strongly special- 

 ized, and have thus become unable to produce germ-substance ; 

 for the leaf-cells in these different plants have hardly under- 

 gone histological differentiation in different degrees. If, not- 

 withstanding, the one can produce a flowering plant, while the 

 others have not this power, it is of course clear that reasons 

 other than the degree of histological differentiation must exist ; 

 and, according to my opinion, such a reason is to be found in 

 the admixture of a minute quantity of unchanged germ-plasm 

 with some of their nuclei. 



In Sach's excellent lectures on the physiology of plants, we 

 read on page 723' — ' In the true mosses almost any cell of the 

 roots, leaves and shoot-axes, and even of the immature sporo- 

 gonium, may grow out under favourable conditions, become 

 rooted, form new shoots, and give rise to an independent living 

 plant.' Since such plants produce germ-cells at a later period, 

 we have here a case which requires the assumption that all or 

 nearly all cells must contain germ-plasm. 



The theory of the continuity of the germ-plasm seems to me 

 to be still less disproved or even rendered improbable by the 

 facts of the alternation of generations. If the germ-plasm may 

 pass on from the egg into certain somatic cells of an individual, 

 and if it can be further transmitted along certain lines, there is 

 no difficulty in supposing that it may be transmitted through 

 a second, third, or through any number of individuals produced 

 from the former by budding. In fact, in the H3'droids, on which 

 my theory of the continuity of the germ-plasm has been chiefly 

 based, alternation of generations is the most important means 

 of propagation. 



II. The Significance of the Polar Bodies. 



We have already seen that the specific nature of a cell de- 

 pends upon the molecular structure of its nucleus ; and it 



' English translation, by H. Marshall Ward. Oxford, 1887, Clarendon 

 Press. 



