S4 



BOTANY. 



, 40.— Formation of Chemically Different Layers. A stiU 

 iurther differentiation may take place in the thickened wall, 

 by which it comes to be made up of layers which differ 

 chemically from one another. This is brought about by 

 the subsequent infiltration of diverse materials into different 

 layers. In some cases the chemical 

 change is accompanied by so great a 

 physical change that the wall sepa- 

 rates readily into two or more plates.* 

 Thus, in pollen-cells, the original wall 

 is usually differentiated into two wide- 

 ly differing plates : (1) an outer thick 

 cuticularized covering (the extine), 

 and (3) a thin inner membrane (the 

 intine) ; the inner plate is shown by 

 tests to be composed of pure cellulose, 

 while the outer one is generally so 

 filled with other materials as to hide 

 completely the cellulose. 



A similar differentiation of the wall 

 takes place in certain spores, and in 

 such case the outer plate is called the 

 exospore (or epispore), and the inner 

 one the endospore (see C, D, E, F, 

 Kg. 180, p. 262). 



The outer walls of the epidermal 

 cells of many plants show a remark- 

 able separation into one or more 

 plates, the outermost of which is 

 highly cuticularized. In some cases, 

 as in the cabbage, for example, this 

 outer plate may easily be separated as 

 a continuous pellicle — the so-called 

 cuticle. 

 Wood-cells frequently show a well- 

 marked separation into plates. This may be seen in Pintis 

 sylvestris (Pig. 16, p. 26), where there are three such 



Fig. 34. — Striation of the 

 bast fibres of Eaya carnosa; 

 ■a and b, crossed annular stri. 

 atlon ; c, d, e, varieties ot sim- 

 ple annular B'liation.— After 

 Sachs. 



* These are the " Schalen " of Sachs, translated " Shells " In the Eiig. 

 lish edition of his " Lehrbuch." 



