208 THE MOVEMENTS OF FLUIDS IX STEMS. 



ance they undergo the changes in composition known as metas- 

 tasis. The water concerned in the slow current may be derived 

 directly from the surrounding water, as in Elodea, or may enter 

 by the roots or root hairs, or may be taken from the water in the 

 rapid current. 



IY. The stem as a reservoir of nutrient matter. 



The stem mainly serves as a channel for the conveyance of the 

 rapid current concerned in transpiration, and in assimilation, and 

 in the formation of new constructive materials from inorganic 

 elements, namely, by the decomposition of the carbonic acid of the 

 air, the fixation of the carbon by its union with the elements of 

 water, and the formation of a carbohydrate, such as starch. In 

 addition there is the slow current ; the assimilated matter is 

 carried to growing parts or to reservoirs, where the constructive 

 matters are stored up xuitil they are wanted during .growth. 



As the leaves unfold they are supplied with water, large 

 quantities of water being an essential of growth. The water 

 enters by the root and contains the necessary mineral ingredients 

 of plant food in solution. By assimilation in the chlorophyll-bearing 

 cells of the leaf, the leaf itself increases in size until full grown, 

 hence anything interrupting or interfering with the process of 

 assimilation will dwarf the growth of the leaf and young inter- 

 nodes. When the leaf is full grown, and most of the parts have 

 ceased to grow in length, although circumferential growth still 

 goes on in the stem, much of the nutrient matter formed by 

 assimilation is produced only to be stored up in a reservoir. By 

 the slow current this material is carried to the medullary rays of 

 the wood to the wood prosenchynia, and also to much of the 

 cellular ground tissue of the stem and shoots, and is there stored 

 up. Hence when the leaves fall in autumn there is a large store 

 of nutrient matter, such as starch, in the cells of the stem. 

 During the winter the cells of the stem are full of fluid and 

 reserve matter. Thus, if a branch be cut of in winter, and 

 exposed to the alternations of sun heat and cold in the early mild 

 days of spring, the sap, i.e., water with constructive matters in 

 solution and suspension, exudes, owing to the admixture of atmo- 

 spheric air or gases with the fluid contents of the cell. The heat 

 expands the air and forces the fluid out ; the process, however, 

 ceases whenever the temperature sinks. Then in the case of the 

 bleeding of trees, in which sap exudes from injured portions of 



