XXIV. ORGANS OF VEGETATION 



THE principal parts of the plant are the root, stem, leaves, 

 flowers, and seed; each has its own special work to do in the 

 development and growth of the plant. The first three are usually 

 designated as organs of nutrition and the last two as organs of 

 reproduction. 



The Root. The root grows downward and gives the plant a 

 firm support in the soil. If we examine this part of the plant 

 closely, we find that a portion of its surface is thickly covered with 

 very fine rootlets, through which moisture and food elements are 

 drawn from the soil for the use of the plant. These elements 

 must undergo certain changes, however, before they can be appro- 

 priated and used by the plant. From the roots there is an upward 

 current which carries the nutritive element of the soil through the 

 woody parts of the stems to the leaves, where it is changed 

 under the influence of the air and sunlight upon the leaves and 

 made available as plant food. Cambium is the name given by old 

 physiologists to the nutritive juice upon which the plant lives, and 

 the cambium layer is the ring or zone of tender wood forming 

 tissue between the bark and the woody part of the plant. This 

 movement of the sap from the roots upward to the leaves we usually 

 designate as the upward circulation. If we break off the top of a 

 milkweed plant, we find that in a short time the sap movement will 

 cause the accumulation of a large amount of the milky white 

 juice of the plant on its broken surface. When grapevines are 

 pruned in the spring, the exudation of a watery fluid from the cut 

 surfaces is very noticeable. This sap movement is not due alto- 

 gether to capillarity, but depends largely upon what is termed root 

 pressure. By this term is meant the force exerted in supplying the 

 water from the soil. With many plants this force is strongest in 

 the spring and gradually decreases as fall and winter approach. 

 The amount of moisture present in the soil, temperature of the 



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