322 STATE HORTICULTURAL SOCIETY. 



RELATION OF PLANTS TO WATER. 



In connection with the above from Prof. Bessey, it may not be out of place to 

 quotQ briefly from a lecture by Prof. Goodale before the Lowell Institute : 



He said that aquatic plants absorb water through the surface of all submerged 

 parts. Plants fixed in the soil absorb water through the superficial tissues of 

 the youngest roots; and chiefly through root-hairs. Leaves of such plants 

 absorb no moisture, even when wet by rain. When a plant is torn roughly 

 from, the soil, nearly all these root-hairs (which are delicate, elongated cells, 

 thickly clothing a short portion of the youngest roots just behind the root tip) 

 are left behind, and the power of the plant to absorb water In ended. The idea 

 that the tip, or sj)07igiok, absorbs water has been exploded by experiments, as 

 also the idea that when these root-hairs, or the portions of the root which bear 

 them, are torn off, water is absorbed by the wounded part. The whole work 

 (except in the case of coniferous trees, which have no root-hairs, and absorb 

 "water by the newer parts of the root, but never by the tip) of forcing water into 

 the plant, against a pressure of three to five atmospheres, is done by these minute 

 and delicate root-hairs. The manner in which tiiis work is done was then 

 described, as far as possible, as follows: If two liquids of different density are 

 placed in contact, they will become mixed in a short time, by the process called 

 diffusion. If a vegetable or animal membrane is interposed between the two, 

 the diffusion is not impeded but hastened, and this hastened operation is called 

 " diffusion by impulse," or (from the Cxreek) osmosis. This operation is what 

 takes place in plants, although the fact that cndosmosis alone exists in them, 

 "without exosmosis, for a time caused botanists to seek for another solution of 

 the problem. 



The process of root absorption and the transfer of the dilute solution is hast- 

 ened by transpiration, or evaporation from all parts of the plant that are exposed 

 to the air. The amount of daily evaporation from common plants has been 

 approxiniately determined by careful experiments, which show that grasses and 

 similar plants in a hot summer day exhale about their own weight of water. A 

 sunflower three and a half feet high was found to present thirty-nine square 

 feet of evaporating surface, and its roots had the aggregate length of 1,448 feet. 

 The process of transpiration is regulated by delicately balanced double valves, 

 called stomafa,ol which there are 100,000 in a single apple leaf, and which are 

 so sensitive as to close if the leaf is shaken. These valves consist of pairs of 

 cells controlling the opening of minute tubes leading to theinner cells through 

 the outer membrane of the leaves, which is impervious to ordinary transpiration. 



Transpiration affects the plants themselves, the air and the soil. In the plant 

 it hastens the transfer of the juices, and condenses the solutions in the trans- 

 piring cells. The air is both cooled and moistened by it. The soil is relieved of 

 superfluous moisture in some cases, as for instance by the gum tree planted in 

 the Eoman Cumpagna to drain the swampy soil and thereby diminish malaria. 

 It is not probable that plants affect the amount of rain fall in any great degree, 

 but they do act as storers of moisture. This they do partly by shading the 

 ground and checking direct evaporation, and partly by allowing smaller plants 

 to grow in their shade, and by the action of their roots keep tlie soil in a con- 

 dition to retain moisture. Moreover the rain is prevented from running off the 

 surface as soon as it falls, as it will do from a surface not clothed with vegeta- 

 tion. In districts which have been made treeless by human instrumentality^ 

 periods of parching drouth are followed by devastating torrents. Men who 



