HOW PLANTS GROW 



of all plant cells consist chiefly of this carbohydrate. Thus it forms 

 almost the whole of the skeleton or framework of plants. It is built 

 by the plant cells from the simpler carbohydrates — the starches and 

 sugars. The thickness of the cell walls, and consequently the per. 

 centage of cellulose, varies greatly in different parts of plants, the 

 walls being thick and resistant in the woody stems, and thin and 

 delicate in the softer parts, such as the fruits and leaves. Especially 

 in the woody parts of plants, the cell walls do not consist simply of 

 pure cellulose, but of cellulose 

 combined with other related 

 carbohydrates, which are even 

 tougher and more resistant. 

 In analyzing plants the chem- 

 ist includes cellulose and these 

 other compounds under the 

 term fiber. 



The pentoses and pentosans 

 are carbohydrates with 5 atoms 

 of carbon in the molecule, in 

 place of 6 as in the sugars and 

 starches. The pentoses corre- 

 spond to sugars, and the pen- 

 tosans to starch and cellulose. 

 The pentosans are widely dis- 

 tributed in plants, being found 

 in largest amount in the more 

 woody portions and in the 

 outer portions of seeds. While 



corn grain contains less than 6 per ct. of pentosans, straw and hay 

 from the grasses usually contain over 20 per ct. 



Fats and oils. — Fats, which are solid at ordinary temperatures, and 

 oils, which are liquid, are composed of the same elements as are the 

 carbohydrates ; i. e., carbon, hydrogen, and oxygen. In fats and oils, 

 however, the proportion of carbon and hydrogen is greater. They 

 therefore give off more heat on burning, one pound of fat producing 

 about two and a quarter times as much heat as a pound of carbo- 

 hydrates. Oils and fats most abound in the seeds of plants, the 

 reserve food supply in peanuts and flax seed, for example, being 

 largely in this form. 



Nitrogenous compounds. — In the living plant cells sugar and 

 starch, formed from carbon dioxid and water thru the action of the 

 sunlight, are united with nitrates and other salts gathered by the 

 roots from the soil to form a new group of complex compounds called 



Starch Grains 



A, From corn gi-ain; b, from wheat; c, 

 from oats; d, from potato. (Magnified 

 330 times.) 



