3^4 



LECTURE XX 11. 



acts on the existing constructive materials as a centre of attraction ; every reservoir 

 of reserve-materials, and every organ of assimilation, on the contrary, is passive 

 towards a growing portion, or, it may be, behaves as a centre of repulsion. The 

 stimulus to the movements of material, however, is always given by the growth of the 

 young organs. The buds of a tree put forth shoots in the spring by no means because 

 the nutritive sap enters into them, as people are in the habit of saying ; but exactly 

 the reverse — the nutritive matters are set in motion because the buds begin to grow. 



All the considerations in this and the last lectures have chiefly had re- 

 ference to the highly organised so-called vascular plants. But in a few words I will 

 show that the essential points are true also for the simply organised Mosses and 



Algae, down to the most simple unicellular forms. We meet everywhere, even in 

 these lower regions, with the same relations between the metabolism which occurs 

 during growth, and assimilation: the storing-up of reserve-materials in perennial parts, 

 e.g. in the tubers so common in true Mosses, and quite generally in the spores, 

 which, although they consist only of single cells, behave in the main like seeds. 

 Indeed, the germinating spore of a INIoss or of an Alga affords us the simplest 

 possible scheme for the process of germination described previously : under the eye 

 of the observer the reserve-materials disappear in proportion as they are used up 

 for the growth of the germinal tubes, just as the reserve-materials in the plumules of 

 the higher plants travel towards the growing ends of the organs, etc. 



