PHYSIOLOGY 253 



a bud (" eye") and not a twig is inserted under the bark of the stock. The "eye " 

 is left attached to a shield-shaped piece of bark, which is easily separated from 

 the wood when the plants contain sap. The bark of the stock is opened by a 

 T-shaped cut, the "eye" inserted, and the whole tightly covered. Occasionally 

 some of the wood may be detached with the shield-shaped piece of bark (budding 

 with a woody shield). In the case of sprouting buds, the budding is made in 

 spring ; in dormant buds, which will sprout next year, in summer. 



The Phase of Elongation 



For the performance of their proper functions, the embryonic 

 rudiments of the organs must unfold and enlarge and assume their 

 characteristic appearance. This subsequent enlargement of the 

 embryonic organs of plants is accomplished in a peculiar and 

 economical manner. While the organs of animals increase in size 

 only by a corresponding increase of organic constructive material and 

 by the formation of new cells rich in protoplasm, and thus require 

 for their growth large supplies of food substance, plants attain the 

 chief part of their enlargement by the absorption of water that is, 

 by the incorporation of an inorganic substance which is most abund- 

 antly supplied to them from without, and to obtain which no 

 internal nutritive processes are first necessary. 



The absorption of water by living cells does not take place with the same 

 rapidity and without interruption as in the case of porous bodies. Before the cells 

 can take up additional water they must enlarge by actual processes of growth. 

 The water, penetrating the young cells by imbibition or by the force of osmotic 

 pressure, is uniformly distributed through the protoplasm, which fills the cell ; 

 when the protoplasm is already abundantly supplied with water, it is accumulated 

 in vacuoles (Fig. 58), As the vacuoles contain also organic and inorganic matter 

 in solution, they exert an attractive force and give rise to further absorption of 

 water. The sap of the vacuoles would, in turn, soon be diluted and its attractive 

 force diminished, were it not that the regulative activity of the protoplasm soon 

 provides for a corresponding increase of the dissolved salts, so that the concentra- 

 tion and attractive force of the sap are continually being restored or even increased. 

 The separate vacuoles thus enlarged ultimately flow together into one large sap- 

 cavity in the middle of the cell. 



In this process the volume of the cell may increase a hundred- 

 fold (in the internodal cells of the Characeae two thousand-fold) 

 without any marked increase in the amount of protoplasm. The 

 enlargement of the cell has been almost wholly produced by the 

 increased volume of water in the sap cavity, which, to distinguish it 

 from the "nutrient water," "imbibition water," and "constitution 

 water" of the plant, may be designated "distension water." 



As is observed in a whole series of vital phenomena, the rate 

 of distension of the walls with the distension water is not uniform, 



but BEGINS SLOWLY, INCREASES TO A MAXIMUM RAPIDITY, AND 

 THEN, GRADUALLY DIMINISHING, ALTOGETHER CEASES. As all the 



