342 



GENERAL PRINCIPLES 



enough lo expand wilh the growth of its contents. But there 

 are often rigid, supporting or protecting structures, which do 

 not permit free expansion of the body. In such cases there are 



many interesting devices employed 

 for securing expansion. The spHt- 

 ting bark of the exogen type of stem 

 has been described, as well as the 

 growing root-tip and the bud. If 

 the position of the apical cell and the 

 angles of the planes in which the suc- 

 cessive segments are cut off from it 

 are carefully considered it will be 

 seen that we have here also a device 

 for securing freedom for growth. Let 

 us consider a few more cases among 

 plants. The diatoms are always 

 unicellular, and each cell is encased 

 in a silicious capsule. The substance 

 of which this capsule is composed is 

 absolutely unyielding so far as the 

 growth of the living contents is con- 



FlG. 



f^rowlh 



of 



212. — ^Mechanics 

 .1, A diatom; 

 Microspora; C, (Edogonium. 

 In .1; a, the silicious "pill-box" 

 shell of a diatom; b, a diatom 



ihe old one. In B: a, b and r, of two parts, which fit into each other 



three steps in the process of im .i . r i ,• 



forming a new cross wall and ^iKC the parts of a common gelatin 

 elongating the side walls of a capsule or a pill box, and can slide 



(hviding cell. In C.- a, the cir- . 



cLilar pad formed within the old apart as the protoplasmic Contents 



r;l''Xwa,r^piruTlefthe i-l^^ase in volume. At the time of 



pad and the pad stretching to division a new half capsule is formed 



I orm new wall; c, ridges left by . ., i, r .i u t_ ir i 



a repetition of the process. inside each ot the old half capsules. 



This means that each generation is 

 confined in a slightly smaller compass than the preceding. 

 Finally a limit is reached beyond which this decreasing size 

 will not go. The shell is then cast off completely, a brief 

 period of rapid growth as a naked cell ensues, and then a 



