THE THALLUS OF THE HIGHER ALG.E. 35 



form. At the a[)ex 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. 

 One 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 (i, 2, 3, fig. 44). 

 The cells so produceci 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) meristem. 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 |f|f||jmMffMi 

 at the ai)ex. Ihe cells nearer the sur- aws^egw;**^®*®®"' 

 face are smaller and of different form <?x*' "'=»'*<?«• "^ 

 from those in the interior. They are « cnijtf^^^'^^^ssia, 



also close-set, whereas those in the in- 0,^53=0= "^^^^ 

 terior are no longer in contact with each '^^^^^^ cx'^^^^'^ 

 other on all sides, but have been sep- f±^SS^<s 



arated by the growmg of branches from |g||g|^^^* ' 

 the cortical cells between them. These 



rid. 45 — Diagram of a por- 



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



about 70 diam c corte.x; 

 terlaced, with wide intercellular SpS.neii. ;«, medulla. The varied 



forms of the cells are due 



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



■" which the filaments are 



and, instead of being filled with prOtO- cm- The dear spaces are 



filled with mucilage pro- 



nlasni, they will be found to have large duced by the ceil-waiis. 



t ' -^ From a drawing by Mr. 



vacuoles and heterogeneous contents. c. e. Alien. 

 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 



