CIRCULATION. 155 



of the leaves, thence by the leafstalks into the liber, and so pervading 

 all, down to the extremities of the roots. 



798. On its passage it makes deposits of food, first in the cells, 

 of the pith at the base of every incipient bud ; then in the cambium 

 region a copious store ; next in the medullary rays a due portion, 

 some carried outward for the supply of the cortic# layer, and some in- 

 ward for solidifying the wood ; and lastly, the residue, often the richest 

 legacy of all, falls to the root, and fills every branch and fiber, however 

 vast its extent. This last deposit is that which is first met and dissolved 

 by the rising tide of fluid in the following spring. 



799. Growth progresses downward. Since the flowing of the 

 true elaborated sap is downward, it scarce admits of a doubt that the 

 progress of the growth is also downward, from the leaves to the roots. 

 And on no other supposition can we account for the results of the fol- 

 lowing 



800. Experiment. Gii-dle an exogenous tree by removing an entire ring of ita 

 bark. It will flourish still during one growing season, and form a new layer of wood 

 and bark everywhere above the wound, as before, but not at all below. The next 

 season the tree will die. Why ? Because the true sap returning can not descend 

 to nourish the roots. 



801. Exp. If a ligature be bound firmly around a stem (se. of silver-leaf poplar) 

 its growth is checked below, while the part just above will exhibit, after a year or 

 two, a circular swelling evidently caused by the interruption of the descending sap. 



802. Exp. If a chip be cut from the trunk, the wound heals evidently from the 

 upper side. 



803. Exp. Cut off the top of a branch just below a leaf. The upper remaining 

 intemode will perish. It has no leaf above it to send down its food. 



804. Exp. Grirdle carefully the stem of a potato-plant. No tubers wiU be formed 

 below. And, again, girdle a fruit tree, and the fruit will for once be increased in 

 amount. 



805. In a few instances trees have survived the girdling process. In 

 such cases the medullary rays complete the broken currents. The descending sap, 

 on arriving at the ring, flows inwardly by the medullary rays, making a circuit, and 

 appears again in the bark below the Interruption. 



806. Rotation. Beside this general circulation of fluids rising and 

 falling from extremity to extremity, there is also a special circulation 

 going on pretty constantly in each new cell, called rotation. 



807. Rotation is a flowing of the protoplasm in slender and devious 

 currents on the inner surface of the primordial utricle, rendered per- 

 ceptible by the opaque particles floating in it. The cytoblast also par- 

 takes of the movement. It is well observed in the hairs of Tradescantia, 

 leaves of Vallesneria, and especially in the stems of Chara, where the 

 current expands into an entire revolving layer of protoplasm. It is a 

 vital movement. 



