VEGETABLE CYTOLOGY 
65 
functions required of it. Thus, in the case of outer covering cells as 
epidermis and cork, whose function is that of protecting the under- 
lying plant units, the walls become infdtrated with cutin and 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 sclerenchyme 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, hard- 
ness, and in the case of sclerenchyme 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 muci- 
laginous 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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