THE PLANT: ITS STRUCTURE, OPE - PROCESSES AND ENVIRONMENT 



13 



exception of carbonic acid, be dissolved in distilled 

 water, plants can be grown in the solution and 

 will produce mature seed ; but if any of the sub- 

 stances be lacking in the solution, except carbonic 

 acid, growth will soon cease. All these substances 

 are present in the soil, together with others of little 

 or no value, as alumina, silica and others, but in 

 order that the plant may absorb them they must be 

 dissolved in the soil-water. Most of them exist in 

 the soil in compounds that are but little soluble in 

 water. The soil- water contains carbonic acid, derived 

 principally from decaying organic matter, which 

 has a decidedly solvent action. In addition, the root 

 constantly excretes carbonic acid, which dissolves 

 the plant-food within its reach. By the excretion of 

 acid, roots may etch polished marble surfaces ; and 

 they impart to distilled water an acid reaction. 



Roots of many members of the pea family supply 

 themselves with nitrogen from the air by means of 

 the bacteria which inhabit tubercles on their roots. 

 Roots of forest trees frequently 

 make use of decaying matter by 

 means of fungi, which grow in 

 close contact with them. 



The leaf. — The seed-leaves are 

 commonly gorged with food, con- 

 sisting of proteids (nitrogenous 

 substances, like white of egg), 

 fats, oils, sugar and starch. This 

 food is mostly manufactured in 

 the foliage leaves. 



When starch is heated it sepa- 

 rates into water and carbon di- 

 oxid (CO2). Evidently it may be 

 formed by causing these two 

 substances to unite. This is just 

 what the foliage leaf brings 

 about. It is supplied with water 

 by the activity of root and stem, 

 and it absorbs carbon dioxid from 

 the air. By utilizing the energy 

 of the sunlight the leaf is able to 

 break the bond of union between 

 the carbon and the oxygen of the 

 carbon dioxid, thus leaving the 

 carbon free to combine with water and so to pro- 

 duce starch, and the oxygen free to escape into 

 the air. The energy used in this process is set free 

 again if the starch be burned, either by ordinary 

 combustion or by the slower combustion that takes 

 place in plant or animal cells. All elaborated foods, 

 such as proteids, fats, oils and sugars, yield up their 

 stored energy in the same way. 



In order to make as much starch as possible, the 

 leaf must expose the greatest possible surface to 

 the sunlight and air, but in so doing it runs the 

 risk of losing too much water by evaporation. To 

 meet this difficulty, it has devices that enable it to 

 increase or diminish evaporation (transpiration) 

 according to its needs. Its surface is made water- 

 proof by waxes, varnishes and resins, so that water 

 can escape only at the pores or stomata that are 

 thickly scattered (Fig. 30) over one or both of its 

 surfaces, — as many as 3,500 per square inch in some 

 instances. A section through a stomate is shown in 



Fig. 31, and a diagram of a stomate in Pig. 32. In 

 the guard-cells, which surround the stomata, the 

 plant possesses automatic devices of wonderful 

 efficiency for regulating transpiration. When the 



Fig. 28. 

 Parts ol a young 

 plant, r, root- 

 hairs; A, hypo- 

 cotyle, between 

 the seed-leaves 

 and the root: c, 

 seed-leaves or 

 cotyledons ; Z, 

 true leaves. 



rA ap 



Fig. 29. Cross-section of a root, as it grows in the soil, show- 

 ing the lelations of the root-hairs (r/i) to the soil particles 

 isp) and the air spaces (a) ; this soil is represented as con- 

 taining the maximum amount of water compatible with 

 good plant-growth, 



water-supply is abundant, especially in the presence 

 of sunlight, the guard-cells absorb water and ex- 

 pand. The pressure causes the walls that bound the 

 pore or stomate to curve away from each other, thus 

 causing the stomate to open. This is due to the fact 

 that these inner walls are thicker than the outer 

 walls. The effect is the same as would be produced 

 on a rubber tube by thickening one side by cement- 

 ing an extra strip of rubber on it. If such a tube 

 be closed at one end while air or water is pumped 

 in at the other, it will bend so that the thickened 

 side becomes concave. 



The absorption of the water by the guard-cells 

 is aided in sunlight by the action of the chloro- 

 phyll grains which they contain ; these produce 

 sugar, which aids the cell in taking up water from 

 the other cells of the epidermis that have no chloro- 

 phyll grains. 



When, therefore, the water-supply is sufficient, 

 and especially when sunlight, temperature and 



Fig. 30. Stomates of 

 geranium leaf. 



Fig. 31. Stomate of ivy, showing 

 compound guard-ceUs. 



other conditions are favorable for leaf activity, 

 the stomata open and permit the leaf to absorb 

 carbon dioxid. On the other hand, lack of water 

 and unfavorable conditions cause them to close. 



