THE VEGETATIVE ORGANS OP PLANTS. 283 



The final result of the filling up of the cells of the heart- 

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

 passahle to sap, so that the interior wood may be removed 

 hy 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 cviriously 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, ar- 

 ranged upon a skeleton or net-work of fibers and ducts. 

 It is directly connected with, and apparently proceeds 

 from, the cambial-layer of the stem, of which it may, ac- 

 cordingly, be considered an expansion. 



In certain plants, as the cactus (prickly pear), 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. 



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