34 



BOTANT. 



frequently happens, however, that the thickening is in bands 

 of greater or less width, occasionally extending over nearly 

 the whole inner surface. 



One of the simplest cases is represented in Pig. 13, where 

 new material has been added to all parts of the wall ex- 



j-f 



Fis. 135. 



Fio. ViA. 



Fig. 14. 



Fig. 13. — A J optical section of a eclerencbyma-cell from beneath the epidermis of 

 the underground st' m of Piens aquUina, isolated by Schulze'i? maceration The 

 wall consists of an inner very dense layer, and a central less dense one enclosed 

 between two denser ones; these layers are penetrated by pit cliannt'ls, which are 

 seen in the further wall in transverse section. S, a similar cell, more thickened. 

 The pits are here long cana's, which are more or less branched. X about 550. — 

 Afti r Sachs. 



Fig. 14.— Brown-walled cells in the stem of Pleris aquilina. A, a half cell iso- 

 lated and rendered coloi less bv Schiilzs's maceration. .B, a piece moie strongly 

 ningnifled(x 560). The fissure-like pits are crossed, i.e., the fissure is twisted as 

 the ih-'ckening increases; p, a side view of a fissure appearing as a simple channel, 

 since it showb the narrow diameter. C", cross-section; a, boundary lamella; b, €, 

 inner lam^ Use.— After Sachs. 



cepting in small isolated spots. As t\ie wall thickens around 

 these spots, they become at first pits, and finally channels. 



29. — In some cases the pits or channels are simple, 

 straight, or slightly bent extensions of the central cell-cav- 

 ity ; in others they may be branched, as shown in Fig 13Bj 

 in cross-section they may be round, as in Fig. 13.4, or elon- 



