306 HOW CROPS GROW. 



The final result of the filling up of the cell of the 

 heart-wood is to make this part of the stem almost or 

 quite impassable to sap, so that the interior wood may be 

 removed by decay without disturbing the vigor of the 

 tree. 



Passage of Sap through the Stem. — The stem, 

 besides supporting the foliage, flowers and fruit, has also 

 a most important office in admitting the passage upward 

 to these organs of the water and mineral matters which 

 enter the plant by the roots. Similarly, it allows the 

 downward transfer to the roots of substances gathered 

 by the foliage from the atmosphere. To this and other 

 topics connected with the ascent and descent of the sap 

 we shall hereafter recur. 



The stem constitutes the chief part by weight of many 

 plants, especially of forest trees, and serves the most im- 

 portant uses in agriculture, as well as in a thousand other 

 industries. 



§ 3. 



LEAVES. 



These most important organs issue from the stem, are 

 at first folded curiously together in the bud, and after- 

 wards expand so as to present a great amount of surface 

 to the air and light. 



The leaf consists of a thin membrane of cell-tissue 

 directly connected with the cellular layer of the bark, 

 arranged upon a skeleton or net-work of fibers and ducts 

 continuous with those of the inner bark and wood. 



In certain plants, as cactuses, there scarcely exist any 

 leaves, or, if any occur, they do not differ, except in 

 external form, from the stems. Many of these plants^ 

 above ground, are in form all stem, while in structure 

 and function they are all leaf. 



In tlie grasses, although the stem and leaf are distin- 



