THE PLANT IN ACTION. 253 



EXEKCISE LXXVII. 

 The Circulation of Plants. 



Although the movement of sap is not, like the 

 flow of blood in animals, along a definitely traceable 

 system of vessels, yet, in the larger plants, experi- 

 ments show that it passes upward by one route and 

 downward by another. In woody dicotyledons, the 

 crude, or ascending sap, rises inside the cambium, 

 and chiefly through the woody bundles of the outer 

 circles of wood, hence called sap-woody the inner por- 

 tion of the tree, or heart-wood, having become so 

 solid as to obstruct its passage. You may find proof 

 of this in many ways. If you remove a ring of sap- 

 wood from the stem of a tree, its branches wither 

 and die, while hollow trees may flourish, and carry 

 on all the processes of life. If you observe trees that 

 have been cut down in spring, you can easily see in 

 what portions the sap is most abundant. This crude 

 sap may be obtained in spring, by making incisions 

 into the sap-wood, from which it will trickle, or some- 

 times even flow in streams. It is nearly colorless, and 

 tastes of the substances it has dissolved from the tis- 

 sues of the tree. In monocotyledons, the rising sap 

 has a much freer and wider course along the scattered 

 bundles of fibro-vascular tissue. 



The elaborated, or descending sap, passes along 

 the inner layers of the bark, and furnishes the cam- 

 bium with material for the growth of cells, and nour- 

 ishment for the young buds in the axils of the leaves. 

 You may stop its descent by removing a ring of bark 

 from the stem or branch of a tree or shrub, but no 

 wood will be formed below the mutilation. The 



