For March, 1921 



515 



A Lesson on Plant Physiology and the Plant in | 



Relation to Its Environment | 



Being One of a Series of Lessons of a Home Study Course on Gardening. Appearing Regularly in The Gasdenebs' Chronicle 1 



Under the Direction of ARTHUR SMITH i 



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ALMOST immediately after the plantlet emerges from the 

 soil into the light it assumes a green color. This is due 

 to the formation of chlorophyll, which is the name given to 

 the green coloring matter of plants and it can only be formed in 

 light. If we prevent the access of light to a green plant, or to 

 part of it, as in the earthing-up of celery, the green color disap- 

 pears. Without this chlorophyll no formation of food can take 

 place, and the first step in food formation is the absorption of 

 energy from light by the chlorophyll; as a matter of fact this 

 energy comes from the sun and is used by the chlorophyll to 

 break up or take to pieces, the carbon dioxide absorbed from 

 the air and water with inatter in solution, absorbed from the soil 

 by the roots ; these are all recombined iiTto foods for the use of 

 the cell protoplasm in making new parts and repairing waste ; 

 the formation process being known as photosynthesis, literally, 

 "putting together by light." 



Not until this food manufacture commences, and the assimil- 

 ation of it carried on, can new plant substance be formed. Dur- 

 ing germination, new cells containing new protoplasm, are formed 

 in the plantlet from the food supply of the seed, but until chlo- 

 rophyll appears and photosynthesis begins, the entire plantlet, with 

 whatever may remain of the seed, weighs no more when dried than 

 the seed weighed previously to being sown. 



The above work is carried on by the leaves, and tht main 

 function of these may be said to be that of food preparation. 



If we examine an ordinary leaf, it is seen to consist of a green 

 substance through which a network of veijis is distributed. The 

 larger veins send off sinaller veinlets, some of the latter being in- 

 visible to the eye. The vein system of leaves, known as venation, 

 is very diverse, but practically every genus, as well as many 

 species, have a more or less fixed system of venation, so much so 

 that in the majority of cases the genus at least a plant belongs to 

 can be ascertained by an examination of its leaf. In some cases 

 a single very prominent vein, known as the midrib, runs through 

 the middle of the blade, from which all the minor veins arise as 

 branches. These veins are part of the conducting system of the 

 plant, although there is no true circulation in plants analagous to 

 the circulation of blood in animals. In addition to acting as con- 

 duits for water and sap, the veins act as supports for the leaf 

 tissue. 



The upper and under surfaces of the leaf are covered by a 

 delicate and transparent skin {epidermis), which itself has no 

 green color. Examined under a powerful microscope this skin is 

 seen to be made up of very small cells. Each cell is bounded by 

 a wall, and in the epidermis they fit closely together and in soiue 

 cases dovetailing w'ith one another. Minute openings, many 

 times smaller than would be made by the point of the finest sew- 

 ing needle, will be discovered in very great numbers. These open- 

 ings are known as stomata. and give passageway into the in- 

 terior of the leaf, putting the internal cells into communication 

 with the air outside and so facilitating the interchange of gases 

 Plants vary considerably in the number of stomata contained on 

 a square inch of leaf surface, and as a rule there is a much larger 

 number on the under side than upon the upper ; in fact some species 

 have no stomata at all on the upper side ; but at the same time there 

 are a few having scarcely any openings on the under side, the 

 water lily having none at all. The number of stomata found upon 

 a given area of a given species differs, however, according to the 

 enviromnental conditions, the quantity varying with the moisture 

 content of the soil and air, light, temperature, etc., and laboratory 

 experiments have shown that a thirty per cent, increase in the con- 

 tinuoiJs moisture content of the soil wi'l produce an increase of 

 fifty-five per cent, in the number of the stomata. 



Situated at all the openings of the stomata there exist what are 

 known as guard-cells, by which the stomata are opened and 

 closed according to existing conditions, the principal factors con- 

 nected with this movement being the humidity of the atmosphere 

 aiid the amount of moisture the plant is obtaining from the soil. 

 Obviously the closing of the stomata will reduce the amount of 

 water transpired, and this closing will therefore take p'acc when 

 the atmosphere is dry ; at the same time, if there is sufficient water 

 available for the plant, the stomata may remain open even when 

 the atmospheric humidity is extremely low. 



With a combination of conditions causing the stomata to re- 

 main closed for any length of time, harm may result to the plant 

 not only from the deficiency of moisture but because the closing 

 of the stomata will reduce or prevent the activities of the leaf 

 in other directions being carried on. There is no doubt whatever 

 that the activity in all the operations connected with plant life, 

 such as respiration, food manufacture, assimilation and growth 

 can only exist when the stomata are open and the passageway 

 into the interior of the leaf free from obstruction. From this 

 we understand why it is quite exceptional for a plant to remain 

 really healthy for any length of time in a dwelling house. The 

 dry atmosphere of the living room causes the stomata to be al- 

 ways more or less closed ; and not only this but the stomata are 

 liable to be continually closed up by dust. Therefore the reasons 

 why the weekly bath for house plants is so beneficial, and for large 

 leaved plants, why frequent cleaning with a moist sponge is so re- 

 freshing to them and docs so much in the way of keeping them 

 healthy, are apparent. 



While the wilting of leaves is brought about by the amount 

 of water transpired by them being greater than that supplied 

 to the plant through the roots, there appears little doubt that 

 the wilting is also a further effort on the part of the plant to 

 reduce transpiration by decreasing the leaf surface directly ex- 

 posed to the atmosphere. This also explains why leaves of broad- 

 leaved evergreens. Rhododendrons for example, curl or roll up 

 when a period of severe frost as well as of Summer drought 

 prevents moisture being brought up in sufficient quantity to off- 

 set the excessive transpiration. The check to root action which 

 is brought about more or less in the operation of transplanting 

 also causes wilting of foliage, and when, owing to faulty methods 

 in carrying out this operation, the work of the leaves is stopped 

 for a prolonged period, death of the plant is almost certain to 

 ensue. 



Plants which are commonly capable of maintaining them- 

 selves under dry and desert conditions have their leaves and 

 structure modified in a variety of ways to bring them into 

 harmony with such an environiuent. In such regions each leaf 

 endeavors to expose as small a surface as possible in proportion 

 to substance to the dry air and intense light, for it may be 

 mentioned that the extremely bright light prevailing in some 

 regions would be fatal to many plants even were the luoisture 

 conditions favorable. That this reduction in the size of the 

 leaves holds a direct relation to such an environment is evident 

 from the fact that the same species often produces small leaves 

 in a dry region and larger ones in a moist climate In the 

 case of a group of Cacti growing in the desert parts of the South- 

 west, the leaves have become so much reduced as to be no longer 

 used, and the process of photosynthesis is carried on by the globu- 

 lar, cylindrical or fiattcned stems. In some species as with the 

 Yucca, transpiration is reduced by the stomata being deeply sunk 

 into the epidermis. To the same end we find upon the leaves 

 of many plants a growth of hairs or scales, which may form only 

 a slightly downy covering, or the leaves may be covered by a 

 woolly or felt-like mass, so that the epidermis is entirely concealed. 



The skin or epidermis may be regarded as an ever-present 

 check against transpiration, for without it the active cells would 

 soon lose all their water. Some plants have their power of re- 

 sisting desert conditions increased by what may be regarded as 

 several epidermal layers, and also a layer known as cuticle is 

 formed by the exposed walls of the epidermal cells which, being 

 constantly renewed from beicaih. becomes very thick and almost 

 leathery, and this is a constant feature of drought-resisting plants. 

 .Also, most plants inhabiting dry regions have facilities for the 

 storage of water, so luuch so that water may be squeezed out 

 of them in sufficient quantity to relieve thirst. This water is 

 drawn up by the roots which with plants of this character in- 

 variably go down very deeply, in the case of the Mesquitc for e.x- 

 ample, the roots have been found forty feet below the surface. 

 The water stored in the leaves is drawn upon by the plant during 

 the daytime when otherwise the roots could not bring it up fast 

 enough for the plant's needs. 



Between the two epidermal layers covering the upper and lower 

 portions is a mass of tissue making up the body of the leaf. Tliis 



