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 lignin 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. 
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