BOTANY 



PART I 



by apposition, become further thickened and otherwise modified by 

 a process of intussusception. Three distinct layers can frequently be 



Fig. 72.— Strongly thickened cell from the pith 

 of Clematis I'italba. m. Middle lamella ; i, inter- 

 cellular space ; t pit ; w, pitted cell wall in 

 surface view, (x 300.) 



Fig. 73, 



Part of a sclerenchymatous fibre 

 from Vinca major. The striatioiis of 

 the outer layers are more apparent than 

 those of the inner layers. The thick- 

 ness of the wall, as seen in optical 

 section, is also shown, (x 500.) 



distinguished in strongly thickened cell walls, such as those of the 

 wood, a primary, a secondary, and a tertiary thickening layer ; these 



differ in their optical appearance 

 and their chemical composition. 

 The secondary thickening la3^er is 

 usually the most strongly devel- 

 oped, and forms the chief part of 

 the cell wall. The tertiary or 

 innermost layer is usually more 

 highly refractive. 





I 



r 



Fio. 74. — Surface view of cells from the 

 sensitive side of the tendril of Cucurhita 

 Pepo, showing tactile pits, s. (x 540.) 



FiQ. 75.— Transverse section through similar 

 cells to those in Fig. 68 ; a small crystal of 

 calcium oxalate («) is present in the tactile 

 pits, (x 450.) 



Thicker cell walls or layers of the wall which appear homogeneous frequently 

 exhibit a stratification when treated with strong acids or alkalies. In many cases 



