98 A TEXTBOOK OF BOTANY [Ch. Ill, 13 



chemical transformations are collectively designated as the 

 plant's metabolism. Functionally, the metabolic changes 

 center chiefly in the provision of materials serving five ends, 

 — the skeleton, reserve foods, living protoplasm, special se- 

 cretions, and respiration. 



1. THE PLANT SKELETON. In the great majority 

 of plant cells, a part of the food sugar is used in building the 

 cell walls (page 41), which collectively constitute the plant 

 skeleton. The substance of the walls is primarily cellulose, 

 a transparent, elastic, water-absorbing material, of which 

 the filter paper of laboratories is a good illustration, though 

 cotton and linen are nearly as pure. Chemically its formula 

 is (C 6 Hio05) n , which means that its molecule is composed of 

 the combination C 6 Hi O 5 repeated an unknown number of 

 times. The combination C6H10O5 (not known to occur by 

 itself) differs only slightly in proportions from the food sugar 

 (C 6 Hi 2 0g — H 2 = C 6 Hi O 5 ), and is clearly transformed 

 therefrom. The ease with which cellulose absorbs and trans- 

 fers water has high physiological importance in the interior 

 of the plant, but would be fatal on the exterior in contact 

 with dry air. In these outer walls, however, a part of 

 the sugar (or cellulose) is converted into new substances 

 called cutin and suberin, which are waterproof, and have 

 a faintly brownish color ; and the epidermis which enwraps 

 the soft parts of plants, and the cork which encloses their 

 woody stems, have walls of such cutinized or suberized cellu- 

 lose. Furthermore, this cellulose, while ample in strength 

 for the construction of small plants, is too yielding for the 

 building of large ones, which have to withstand great strains 

 from their weight and the winds. Accordingly, in the 

 trunks of trees and shrubs some of the sugar (or cellulose) is 

 converted into a new substance called lignin, which infil- 

 trates and greatly stiffens the walls without loss of their 

 power to transmit water ; and such lignified walls constitute 

 wood. The shells of nuts, and some coats of seeds, also 

 owe their hardness to lignification. And other modifications 



