140 ORGANS OF VEGETATION 



soil, and the time of the day also influence this force. With the 

 grapevine it gradually increases until noon and then begins to 

 decrease, while in the case of the sunflower the maximum pressure 

 is reached before 10 A.M. 



From the leaves the sap current carries the newly formed plant 

 food downward through the cambium layer to all parts of the plant 

 where nourishment is needed. If we girdle the small twig of a tree 

 and destroy the cambium layer, the downward circulation of the 

 plant food will stop where the bark is cut and will cause a noticeable 

 enlargement of the twig at that point. 



The Stem. We have already seen that the stem and its 

 branches are the channels through which the sap movement is 

 carried on between the roots and the leaves of the plant. In 

 addition to this the stem also bears the leaves, buds, flowers, 

 and fruit of the plant. 



Stems may be classified as (1) erect, when they grow in an 

 upright position; (2) prostrate, when the branches lie prostrate 

 on the ground or nearly so as in the strawberry; (3) climbing, 

 as in the case of .the morning-glory and hop vine, where the stem 

 twines about the support. Other examples slightly different 

 are seen in the grapevine, woodbine, and the common gourd 

 plant. 



The Leaves. If we place a fruit jar over a small plant, we find 

 that in a very short time the inner side of the jar is found covered 

 with small drops of water, which has been given off by the leaves. 

 This process is called transpiration, and the amount of water given 

 off through the leaves in this way is much larger than one would 

 think. It is estimated that more than three hundred pounds of 

 water pass through a plant in the process of transpiration for 

 every pound of dry solid matter assimilated by the plant. 

 An acre of good grass land will exhale ordinarily in well- 

 watered regions not less than thirty hogsheads of water every 

 day. 



The leaves of the plant correspond in a measure to the lungs of 

 animals. The carbon dioxide of the air is the element that feeds 

 the plants, and without it they cannot grow. Although there is 

 only about .03 per cent of this gas present in the air, Draper says 

 that if it were taken away in an instant, the whole surface of the 



