34 



BOTANY. 



40. Formation of Chemically Different Layers. A still 

 further 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 (2) 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, 

 Fig. 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 

 Fig. 34. striation of the outer plate may easily be separated as 



bast fibres of Hoya carnosa ; .. ,,. , ,-, -,-, 3 



a and 6, crossed annular stri- a COntmUOUS pellicle the SO-Called 

 ation ; c. d, e, varieties of sim- , . 



j)le annular s'riation. After CUtlCle. 



Sachs> Wood-cells frequently show a well- 



marked separation into plates. This may be seen in Pinus 

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



* These are the " Schalen " of Sachs, translated " Shells " in the Eng. 

 lish edition of his " Lehrbuch." 



