BOTANY. 



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 Fig. 13, where 

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



FIG. 135. FIG. ISA. FIG. 14. 



Fig. 18. A, optical section of a sclerenchyma-cell from beneath the epidermis of 

 the underground et- m of Ptetis aquilina, isolated by Schulze's 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 channels, which are 

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

 The pits are here long canals, which are more or less branched. X about 550. 

 After Sachs. 



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

 lated and rendered colorless by Schulzs's maceration, li, a piece more strongly 

 magnified (x 550). The fissure-like pits ar* crossed, i.e.. ttie fissure is twisted as 

 the thickening increases; p, a side view of a fissure appearing as a simple channel, 

 since it shows the narrow diameter. <7, cross-section; a, boundary lamella; b, c, 

 inner lamellae. After Sachs. 



cepting in small isolated spots. As the 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 13B ; 

 in cross-section they may be round, as in Fig. 13^4, or elon- 



