GROWTH 



343 



new and larger shell is produced as a starting point for a 

 repetition of the process. 



728. Sometimes cells adhering in filaments are encased in 

 thick tubular sheaths, which become too unyielding to keep 

 pace with the growing contents by stretching. In the case of 

 Tolyptothrix the tube splits at a certain point, the chain of 

 cells breaks, and one end pushes past the other and out through 

 the opening. This produces what is called false branching. 

 Again in Oedogonium the unyielding tube in which the cells 

 are encased is made to expand in a curious fashion. Inside 

 the cell a circular cushion of new cell wall substance is formed 

 against the inner surface of the old wall. This cushion com- 

 pletely encircles the cylinder. Then the old wall breaks opposite 

 the cushion, and this permits the latter to stretch and the cell 

 to elongate, while at the same time maintaining the continuity 

 of the cell wall. In Microspora the wall of the filament is made 

 up of segments which are double-wedge-shaped in longitudinal 

 section. These slip apart as growth proceeds and permit the 

 insertion of new wedges by growth. 



729. Among the Monocotyledons two methods of common 

 occurrence are of special interest. In this group of plants we 

 find the sheathing leaf base a very common type. The sheath 

 completely surrounds and protects the stem for some distance 

 upward from the node. Within this sheath the stem remains 

 for a long time meristematic, after the upper part of the same 

 internode has completed its growth. This device permits the 

 stem to continue growth longitudinally for a long time, but 

 growth in thickness is practically completed when the upper 

 end of the node appears above its sheath. This type is espe- 

 cially characteristic of the long, slender, rapidly growing stems 

 of grasses. A modification of this type occurs in the case of 

 the perennial Monocotyledons like the palms. Here, the 

 growth in height is very slow, and the short internodes are pro- 

 tected for a long time by the basal portions of the leaf stalks, 



