THE THALLUS OF THE HIGHER ALG&. 35 



form. At the apex of any growing branch is found a cluster 

 of angular cells, thin-walled, of nearly uniform size, with 

 abundant protoplasmic contents, and all in close contact. 

 ( >ne of these cells, lying in the center of the group, some- 

 what larger and of different shape from the rest {c, fig. 

 44), is constantly undergoing division, and thus cutting off 

 cells (segments) from its two inner faces (1, 2, 3, fig. 44). 

 The cells so produced undergo further divisions, forming 

 thereby all the cells of which the thallus is composed. This 

 group of dividing cells is present in all the higher plants. 

 It constitutes the " growing point " or, better, the apical (or 

 primary) merislem. The single cell from which all proceed 

 in Fucus is called the initial, or apical, cell. 



43. Differentiation of cells. — But if a thin section of the 

 thallus, from an older part, be examined (fig. 45), its cells 

 will be found very different from those 

 at the apex. The cells nearer the sur- 

 face are smaller and of different form 

 from those in the interior. They are 

 also close-set, whereas those in the in- 

 terior are no longer in contact with each 

 other on all sides, but have been sep- **=> ^5^^^> 

 aratec l>v the ^rowim: oi branches from ^.i,,/.,*^. ■..-»• «••« ■• 



J . ?. , f fe ,. rp , ummmmm 



the cortical cells between them. 1 hese 



Fi<;. 45— Diagram of a por- 



filamentous branches are crossed and in- tion of fig. 44 , magnified 



about 70 clum 1 cortex: 



terlaced, with wide intercellular spaces, w, medulla. Thevaried 



forms of the cells are due 



All of these older cells have enlarged, to the different planes in 



. . which the filaments are 



and, instead Ol being Idled With protO- cut. The clear spaces are 



filled with mucilage pro- 



plasm, they will be found to have large duced by the ceff-waiis. 



From a drawing by Mr. 



vacuoles and heterogeneous contents. C. E. Allen. 

 The walls, also, are no longer thin and homogeneous, but 

 have become thickened and differentiated into at least two 

 layers, the outer of which is capable of swelling enormously 

 in water, while the inner layer retains its usual form. There 



