CHAPTER 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 dictinctions 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. digitafa, 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 unlikc- 

 nesses in their conditions. In Furcelluria and various Alg& 

 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 forming 

 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 inves- 

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

 will be found in Appendix C. 



