PHYSIOLOGY 



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 siugle 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. 



Tlie Bole of Water in the Plnnt. — 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- 

 teniling liackward for a considerable distance, the root- 

 lets are clothed with numerous delicate hairs (shown in 

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



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. Bein^r Tiuni<r<.us jui-I 

 extremely delicate, they come into the cluv^r t.-n. Ii w iih 

 the surface film of water adhering to tin- liii Ic pjn-iirh s 

 of soil, and from such film water tlov iin.i.- n.ulilv 

 satisfy th.ir nepfls than from free soil water. Tiny 

 can cxtr.i.t wat.r until the soil contains only a very 

 smrill I"!- .■■•lit, .11- until it is dust-dry. 



'I'll.' r.i,,i hair, al.s.irb water freely, and during active 

 growili it IS f.,n-, ,1 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 

 by lifiinir a column of the liquid absorbed, and often to a 

 coiisi.li I allr li. IlIiI. In any herbaceous plant it may be 

 test, 1 . ! i:7:t. In the grape vine 36 ft. of water 



nia\ The bleeding of plants is an evi- 



\\ai> I ]- :m iiiiiis absorbed in much greater quantity 

 tliaii 1- r. .| iin.l ni.rcly as a constituent of the plant 

 boily. In lai t, 1.. h.rni one ounce of plant substance it 

 is i~iiiiiaii il tliat 1.V2.5 pounds of water must pass 

 thn.u-h 111.- |ilant. This surplus water passes oft 

 IhrouL'h tlir Icavi-s 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 



PHYSIOLOGY 



1323 



of any plant may be cut off and the 

 a bored cork into a bottle of water, 

 be placed a larger jar or bell-glass 

 a mist will collect on the inside 

 Transpiration is facilitated by dry 

 peratures, movement of the plant, 

 or in dry weather transpiration 

 amount of water absorbed by the r 

 A very slight shower will la frish 

 not because the leaves have al.^.,, 1, 



the 



is therr 



in furii 



Full> 



ingcali 



reach a 



end inserted through 

 Over the whole may 

 , and in a short time 

 walls of the latter 

 air, wind, high tem- 

 etc. If on a hot day 

 is greater than the 



la.ots, the plant wilts. 



I a willed plant, but 



...I »aier. The plant 

 ' .1. ami transpiration 



he routs can catch up 



inspired from a forra- 

 of tons of water daily 

 en meadow-land may 



r 12 hours iu a dry 

 reight will be consid'- 

 due to the loss by 



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 riiigs, 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 (monoootyledonous) it is 

 in the thread-like groups of fibers 

 (hbrovascularbundles)scattered rather 

 irregularly throughout the pith. That 

 the current is always through these 



JM 



To test root pressure. 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 oO per cent in 



