VEGETABLE CYTOLOGY 97 



tain functions required of it. Thus, in the case of outer covering 

 cells as epidermis and cork, whose function is that of protecting the 

 underlying plant units, the walls become infiltrated with cutinand 

 suberin, waxy-like substances, which make them impermeable to 

 water and gases, as well as protect them against easy crushing. 

 Again, in the case of stone cells and sclerenchyma fibers whose 

 function is that of giving strength and support to the regions wherein 

 found, the walls become infiltrated with Hgnin which increases their 

 strength, hardness, and in the case of sclerenchyma fibers, their 

 elasticity also. Moreover, in the case of the cells comprising the 

 testa or outer seed coat of the pumpkin, squash, mustard and flax, 

 etc., whose function is that of imbibing quantities of water, the walls 

 undergo a mucilaginous modification. 



Growth in Area and Thickness. The cell wall when first formed 

 is limited in both extent and thickness. As the protoplast within 

 enlarges new particles are placed within the wall by the process called 

 intussusception. This increases its area. New particles, also, are 

 deposited on its surface which gradually increases its thickness. 

 The latter process is known as growth by apposition. 



