CHAPTEE IV. 



DIFFERENTIATIONS AMONG THE INNER TISSUES OF 

 PLANTS.* 



277. IN passing from plants formed of threads or thin 

 lamina^ to plants having some massiveness, we find that after 

 the external and internal parts have become distinguished 

 from one another, there arise distinctions among the internal 

 parts themselves, as well as among the external parts 

 themselves: the primarily-differentiated parts are both re- 

 differentiated. 



From types of very low organisation illustrations of this 

 may be drawn. In the thinner kinds of Laminaria there 

 exists but the single contrast between the outer layer of cells 

 and an inner layer; but in larger species of the same genus, 

 as L. digitata, there are three unlike layers on each side of a 

 central layer differing from them augmentation of bulk is 

 accompanied by multiplication of concentric internal struc- 

 tures, having their unlikenesses obviously related to unlike- 

 nesses in their conditions. In Furcellaria and various Algce 

 of similarly swollen forms, the like relation may be traced. 



Just indicating the generality of this contrast, but not 



* Students of vegetal physiology, familiar with the controversies respecting 

 sundry points dealt with in this chapter, will probably be surprised to find 

 taken for granted in it, propositions which they have habitually regarded as 

 open to doubt. Hence it seems needful to say that the conclusions here set 

 forth, have resulted from investigations undertaken for the purpose of form- 

 ing opinions on several unsettled questions which I had to treat, but which I 

 could find in books no adequate data for treating. The details of these 

 investigations, and the entire argument of which this chapter is partly an 

 abstract, will be found in Appendix C. 



