PHYSIOLOGY 



PHYSIOLOGY 



1323 



culties are directly traceable to over-watering, or "cold 

 feet," the effect of too much water being partially to 

 prevent aeration. Water plants have adapted them- 

 selves to getting oxygen in other ways, and many bog 

 plants send to the surface special roots for aerating 

 purposes. 



No plant can live without oxygen. In some way or 

 other oxygen must be secured. The more active a plant 

 is, whether in growth or in movement, the more oxygen 

 will it require. Even dry seeds must respire slightly, 

 and in some kinds respiration may be so rapid that after 

 a single season death may ensue. This use of oxygen, 

 whether by the germinating seed or by the growing 

 or assimilating part, is accompanied by the giving off 

 of carbon dioxid, or foul gas. This whole process is 

 respiration; and in its ultimate effects it is similar to 

 respiration in animals. 



The Role of Water in the Plant. Even quicker to 

 manifest itself than the action of suffocation by lack of 

 oxygen is the injury which most plants may suffer from 

 an insufficiency of water. The rigidity of an herbaceous 

 or succulent plant is due largely to its water content; 

 and without a substantial degree of this rigidity, growth 

 would cease and life soon become extinct. The plant 

 pulled up by its roots or cut down, wilts almost imme- 

 diately. The wilting of plants, then, is due to a lack or 

 loss of water supply. 



The way in which the ordinary plant may constantly 

 obtain a quantity of water from the soil is worthy of 

 full discussion. On pulling from the soil a growing 

 plantlet of squash, we find a tap-root and a number of 

 small rootlets. To the latter cling, perhaps, small par- 

 ticles of the soil, as in Fig. 1777. If, however, seeds 

 are germinated between pieces of moist paper or cloth 

 so that there will be no disturbance of the delicate grow- 

 ing parts, further structures will be evident. From a 

 quarter of an inch or so behind the root-tip, and ex- 

 tending backward for a considerable distance, the root- 

 lets are clothed with numerous delicate hairs (shown in 

 Figs. 1778 and 1788) . These are the root-hairs, arid it was 

 to such as these that the soil clung in Fig. 1777. They 

 are simple, long, tube-like cells consisting of a cell wall 

 with living protoplasm and cell sap. The inner proto- 

 plasmic lining of this cell wall permits water and salts 

 in solution to pass inward by the interesting process 

 of osmosis. 



The root-hairs are temporary structures which never 

 grow into rootlets, but which die away as the roots 

 become old or woody. While living they perform the 

 important function of absorbing from the soil nearly all 

 of the water needed by the plant. Being numerous and 

 extremely delicate, they come into the closest touch with 

 the surface film of water adhering to the little particles 

 of soil, and from such film water they more readily 

 satisfy their needs than from free soil water. They 

 can extract water until the soil contains only a very 

 small per cent, or until it is dust-dry. 



The root-hairs absorb water freely, and during active 

 growth it is forced upward into root and stem so vigor- 

 ously that a pressure (root pressure) of considerable 

 extent may be manifest. If the plant be severed and a 

 tube applied to the stump, this pressure manifests itself 

 l>y lifting a column of the liquid absorbed, and often to a 

 considerable height. In any herbaceous plant it may be 

 tested, as in Fig. 1779. In the grape vine 36 ft. of water 

 may be maintained. The bleeding of plants is an evi- 

 dence of root pressure. 



Water is actually absorbed in much greater quantity 

 than is required merely as a constituent of the plant 

 body. In fact, to form one ounce of plant substance it 

 is estimated that 15-25 pounds of water must pass 

 through the plant. This surplus water passes off 

 through the leaves and other succulent parts, princi- 

 pally through the stomata previously mentioned. This 

 process is one of evaporation from living membranes, 

 and it is called transpiration. That transpiration is not 

 merely an evaporation process may be roughly shown 

 by an experiment with two similar leafy branches 

 freshly severed. One of these is dipped in hot water 

 to kill the protoplasm, then the two are left to dry out. 

 Transpiration from the living twig will be less rapid 

 than evaporation from the dead one. The demonstra- 

 tion of transpiration is an easy matter. A leafy branch 



of any plant may be cut off and the end inserted through 

 a bored cork into a bottle of water. Over the whole may 

 be placed a larger jar or bell-glass, and in a short time 

 a mist will collect on the inside walls of the latter. 

 Transpiration is facilitated by dry air, wind, high tem- 

 peratures, movement of the plant, etc. If on a hot day 

 or in dry weather transpiration is greater than the 

 amount of water absorbed by the roots, the plant wilts. 

 A very slight shower will refresh a wilted plant, but 

 not because the leaves have absorbed water. The plant 

 recovers because the air is saturated, and transpiration 

 is thereby so much lessened that the roots can catch up 

 in furnishing the necessary supply. 



Fully a quart of water is daily transpired from a form- 

 ing cabbage head, and the number of tons of water daily 

 given off per acre by forest or even meadow-land may 

 reach an astonishing figure. The amount of water tran- 

 spired by a small potted plant may be readily weighed. 

 Place the pot in a glass jar as seen in Fig. 1780, tying 

 over the top and snugly around the plant some soft rub- 

 ber cloth. Water the plant through the glass tube and 

 then weigh. After standing 6 or 12 hours in a dry 

 atmosphere th'e weight will be consid- 

 : : ;:; erably reduced, due to the loss by 

 transpiration. 



The path of the ascending water 

 current or sap current is through par- 

 ticular vessels of the young woody 

 parts. In trees it ascends in the 

 youngest wood rings, not between bark 

 and wood. In herbaceous netted-veined 

 (dicotyledonous) plants the path is in 

 the ring of woody tissue or bundles 

 between the bark and pith. In the 

 Indian corn (mpnocotyledonous) it is 

 in the thread-like groups of fibers 

 (fibrovascular bundles ) scattered rather 

 irregularly throughout the pith. That 

 the current is always through these 



1779. 

 To test root pressure. 



1780. Experiment to determine the 

 amount of water transpired. 



woody bundles in the above plants may be demonstrated 

 by placing branches of the plants in a tumbler contain- 

 ing some eosin solution. In a few hours the bundles 

 will be colored for a considerable extent. The current 

 will rise much faster if the branch to be used is cut un- 

 der water. This prevents the access of air and the par- 

 tial stoppage of the conducting channels. For the same 

 reason flowers wilt less rapidly when the stems are cut 

 under water. 



The total amount of water in plants varies from a very 

 small percentage in dry seeds to about 50 per cent in 



