232 THE MOVEMENT OF WATER THROUGH THE PLANT 



siderable distance through plants. In general there is no correlation between 

 the pressure with which the sap is exuded from cut stems and the volume of 

 sap flow. In some species relatively large volumes of sap may be exuded 

 under a relatively low pressure, in others exactly the opposite relation may 

 exist. 



Some of the woody plants in which bleeding from cut surfaces occurs, 

 commonly ascribed in part or entirely to root pressure, are the sugar maple, 

 box elder, dogwood, hornbeam {Ostrya), sycamore, birches, currant, and grape. 

 Such exudation of sap under pressure usually occurs only during the seasons 

 of the year when the plant bears no foliage, and most commonly in the early 

 spring months. The popular concept of a "rise of sap" in woody plants in the 

 spring is largely based on the observation of such phenomena. Exudation of 

 sap under pressure can also be demonstrated in many species of herbaceous 

 plants especially if potted plants are used and the soil is flooded with water 

 before the demonstration is attempted. 



The sap which moves through woody plants in the spring and, less abun- 

 dantly, in the autumn under the influence of "root pressures" contains appre- 

 ciable concentrations of carbohydrates as well as traces of mineral salts. In 

 contrast with this is the condition of the xylem sap in the summer at which 

 season it contains traces of mineral salts, but practically no soluble carbohy- 

 drates (Fig. 107, Fig. 109). 



It has sometimes been claimed that "root pressures" are adequate to ac- 

 count for the rise of water in herbaceous plants and in low shrubs or trees, 

 and that they may even play a very considerable part in the ascent of sap in 

 taller trees. While it is undoubtedly true that "root pressure" does, in some 

 species of plants, under certain conditions, account for the movement of some 

 water in an upward direction through plants, this process is inadequate to 

 account for the translocation of any considerable amount of water through 

 plants. In the first place there are many species in which this phenomenon has 

 never been observed. In the second place, the magnitude of the pressure de- 

 veloped is seldom sufficient to force water to the top of any except relatively 

 low-growing species of plants. Neither is the rate of flow, as it occurs in 

 most species, anywhere near rapid enough to compensate for the known rates 

 of transpiration. Finally, and this is perhaps the most fundamental objec- 

 tion to the idea that "root pressures" play a very prominent part in the ascent 

 of sap in plants ; they are usually negligible, in temperate regions at least, 

 during the summer period when transpiration is most rapid. During periods 

 of rapid transpiration, the cut surfaces of plants not only fail to exude sap, but 

 will usually absorb water if it is supplied at the cut surface. 



The trunks of many species of deciduous trees exhibit a marked seasonal 



