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GARDENERS' CHRONICLE 



Water Conservation in Plants 



WILLARD N. CLUTE 



ONE of the first requirements of plants is an adequate 

 supply of moisture. Alany species grow immersed 

 in water and the roots ot land plants ramify 

 through the soil in all directions in search of this precious 

 fluid. In all growing plants, there is a constant stream 

 of moisture flowing in through the roots and out through 

 the stem and leaves. One seldom realizes the amount of 

 water that passes through a plant in this way. The alfalfa 

 plant uses more than a thousand pounds in producing a 

 pound of dry matter and other crops require quantities 

 almost as great. 



It is plain that only a small part of the water that flows 

 into the plant is used in making food. The rest escapes 

 into the air as water-vapor but it is not wasted, for by 

 evaporating it keeps the plant cool. Recent experiments 

 have shown that a leaf in full sunshine may receive 

 enough heat in one minute to raise its temperature thirty 

 degrees. At this rate a fatal temperature would soon be 

 reached were it not for evaporation. Just how eft'ective 

 this process is may be realized from the fact that it re- 

 quires more than five hundred times as much heat to 

 evaporate a given quantity of water as it does to raise 

 its temperature one degree. 



Notwithstanding the constant supply of moisture from 

 the soil, plants are often in considerable danger of dry- 

 ing up. This is especially true of specimens exposed to 

 periods of actual drouth, or to the physiological drouth 

 of Winter when the water in the soil is not available be- 

 cause frozen. When such periods occur the plant usually 

 reduces sail by dropping its leaves. Many desert plants 

 have become practically leafless in response to extended 

 seasons of this kind, their work of food-making being 

 now carried on by the green tissue of the stems. When 

 the drouth is only temporary, as in the hottest part of 

 the Summer day, the leaves may assume diff'erent posi- 

 tions to avoid the strong sunlight and consequent loss of 

 water. Corn leaves roll up, the leaves of compass plants 

 and wild lettuce turn their edges to the sun and the leaves 

 of many of the Lcguminosa droo]i. In cold seasons, a 

 similar rolling of leaves is noticed in most of the plants 

 hardv enough to retain these structures. 



The leaves are peculiarly situated as regards moisture. 

 On the one hand they must evaporate enough to keep 

 cool and on the other they must retain enough to enable 

 them to carry on their activities. They are therefore 

 provided with an epidermis that prevents the passage of 

 moisture, but which is provided with great numbers of 

 tiny openings called stomata. through which moisture 

 may escape. The cells which surround these openings 

 can contract or expand them as occasion requires and 

 thus the loss of water is in a measure controlled. 



Young stems are also covered by an epidermis in which 

 there are openings known as lenticels. The tiny s]iecks 

 seen in the young bark of elder, cherry, ailanthus and 

 other woody plants arc lenticels. In the white birch the 

 increasing girth of the stem stretches the lenticels hori- 

 zontally, forming the familiar dark streaks in the bark. 

 The streaks in a piece of cork are produced by the same 

 structures. Stomata are usually too small to be seen with 

 the unaided eye. but they are visible in the leaves of the 

 common white or I\Tadonna lily and with a simple lens 

 thev may be seen in the leaves of many other plants. 



When leaves are exposed to much sunlight as in the 

 tropics, the epidermis often secretes a layer of cuticle con- 

 sist inp" of a waxv substance called cutin. Sometimes the 

 cut in is in rods, grains or flakes which give the leaves a 



gray or whitish color, as in the cabbage and carnation, 

 ihis layer, called bloom, is characteristic of such fruits 

 as the grape, plum, and nectarine. A similar deposit on 

 the apple makes the fruit shine when rubbed, a fact which 

 has been commercialized by the owners of fruit stands. 

 Sometimes the leaf has what is known as a multiple epi- 

 dermis in which additional cells filled with water aid in 

 absorbing the sun's heat. A very good illustration of 

 this may be seen in a cross section of the leaf of the 

 common rubber plant. 



Hairs and scales of various kinds aid the epidermis in 

 protecting the leaves. Some are simple, others branched 

 and still others have glands at their tips in which is an 

 oil reputed to still further conserve the moisture. The 

 leaves of Deiitsia. mullein, primrose, hollyhock, butifalo- 

 berry and Geranium- afford fine examples of hairs and 

 scales. These are beautiful objects for microscopic study. 

 In the woody parts of perennial plants, the epidermis 

 is sooner or later replaced bv a more serviceable tissue 

 of cork cells which are closely joined together like epi- 

 dermal cells but which are unlike them in containing air 

 instead of living matter. To this fact the lightness of 

 cork is largely due. Cork not only protects the stems 

 from evaporation but, since heat crosses stationary air 

 verv slowly, it protects the plant from the sudden changes 

 of temperature which are far more harmful than steady 

 cold is. The tissue that forms the cork originates m the 

 epidermis, or in the layer of cells just below it, which is 

 known as cork cambium. Since the formation of a layer 

 of cork shuts ofT the water from the epidermis, the latter 

 soon dies and falls away. Often the cork cambium is 

 thrown ol¥ in turn by another layer that forms deeper in 

 the tissues of the bark. 



The old corkv bark may remain on the stem for_ a 

 number of years, but ultimately it is shed, being cracked 

 and loosened bv the expanding tissues within. Owing to 

 the way in which the cambium is arranged, the bark 

 breaks up into patterns that are characteristic and distinct 

 enough to serve for identifying the difterent species. One 

 calls to mind in this connection, the rough bark of old 

 cottonwoods, the fine netted bark of walnut and ash, the 

 winged bark of hackberrv and the splintery bark of 

 hickorv. In the buttonwood or sycamore the bark falls 

 oft' in flakes, in the birch it forms papery layers, and in 

 the grape-vine it forms long strips. Sometimes the cork 

 cambium does not develop evenly and thus gives rise to 

 corky ridges and wings that are often very conspicuous 

 as in the sweet gum. the Euonymus and some oaks and 

 elms. The cork from which the stoppers of bottles are 

 made is the greatly developed tissue of a species of oak. 

 In voung twigs cork cells mav be of various colors and 

 thus give the characteristic colors to many species. The 

 red of dogwood, the yellow, orange and purple of wil- 

 lows, the green of wahoo, the olive of golden bell and 

 the sober gravs of beech and butternut are illustrations of 

 this. 



In addition to its other functions, cork- protects the 

 tissues of the stem from mechanical injury and assists in 

 healing wounds that may occur. When a branch is cut oflf 

 a laver of cork soon spreads over the wound under cover 

 of which the tissues within may complete the repairs. 

 Cork tissue also covers the scars left by the fall of the 

 leaves. In exposed situations the bark of trees is thickest 

 on the side most exposed to the cold and wind. On the 

 other hand, the trees in tropical regions where the air is 

 ( Coiiti'uu'd oil /'(7,C'' 5?\) 



