342 



GENERAL PRINCIPLES 



enough to expand, with 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 split- 

 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 



OU U * seen that we have here also a device 



1 \*\ L-4 for securing freedom for growth. Let 

 0Vl KM I I k us cons ider a ^ ew more cases among 

 Ll ll I I 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- 

 cerned. But the capsule is composed 

 of two parts, which fit into each other 



FIG. 212. Mechanics of 

 growth. A, A diatom; B, 

 Microspora; C, (Edogonium. 

 In A: a, the silicious " pill-box" 

 shell of a diatom; b, a diatom 

 dividing and forming two new 

 half shells, back-to-back, within 

 the old one. In B: a, b and c, 



three steps in the process of ri .-, r i ,. 



forming a new cross wall and llke the P arts of a Common gelatin 



capsule or a pill box, and can slide 

 apart as the protoplasmic contents 

 increase in volume. At the time of 



elongating the side walls of a 

 dividing cell. In C: a, the cir- 

 cular pad formed within the old 

 wall preparatory to elongation; 

 b, the old wall split under the 

 pad and the pad stretching to 

 form new wall; c, ridges left by 

 a repetition of the process. 



division a new half capsule is formed 

 inside each of 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 





