44 



THE GARDENER'S ASSISTANT. 



the oxygen gas is emitted, as is readily proved 

 by simple experiments. The bubbles of gas, 

 which appear on the surface of leaves im- 

 mersed in a tumbler of water and exposed to 

 the light, consist of almost pure oxygen gas. 

 The oxygen, then, which is absorbed in the 

 constant respiratory process is partly utilized 

 and partly eliminated in the presence of chloro- 

 phyll under the influence of light. The carbon is 

 retained in the plant and forms proportionately 

 the greater part of its solid structure. As a 

 consequence of this action liquid sugar and 

 solid starch are formed in the chlorophyll, and 

 these, together with nitrogenous compounds 

 called proteids, constitute the real food of the 

 plant. The elements that have been men- 

 tioned are the ingredients out of which, by the 

 chemical work effected in the protoplasm, the 

 true food is formed. The liquid food, sugar, or 

 nitrogenous compounds, are conveyed from place 

 to placo according as they are required, the 

 solid starch is liquefied by the action of a fer- 

 ment called diastase, and when thus rendered 

 capable of transport it is carried from part to 

 part, and in many cases becomes again insoluble. 

 and is deposited in certain store-cells, as, for 

 instance, in the tuber of a potato. Here the 

 starch is formed in the leaves under the influ- 

 ence of light and heat, and conveyed in a liquid 

 form through the petiole down the stem into the 

 tubers, where it is again solidified and stored 

 for future use. Similarly the nitrogenous com- 

 pounds are rendered soluble by various ferments 

 secreted in the protoplasm, and which convert 

 them into "peptones", which are then available 

 as plant-foods. But these are matters for the 

 chemist rather than the gardener to investigate. 



It may be asked what becomes of the nitrogen 

 which forms so large a share of atmospheric air ] 

 It might naturally be supposed that the nitro- 

 genous compounds found in the living proto- 

 plasm under all circumstances were derived from 

 the nitrogen of the atmosphere. It is, however, 

 a well-established fact that the air supplies 

 directly none of the nitrogen required by the 

 plant, and that the whole is obtained from the 

 soil and enters the plant by the roots, as previ- 

 ously mentioned. Although the leaves and other 

 exposed parts of plants are, as it were, bathed 

 in atmospheric air, it appears that they select 

 the oxygen and the carbonic acid gas of the air 

 and leave the nitrogen. Where and how the 

 decomposition takes place, whether on the surface 

 or within the plant, is not certainly determined. 



As the chief chlorophyll -containing parts of 

 plants the leaves have the greater part of the 



work of the assimilation of carbon to do. They 

 also take a share in the respiratory process. In 

 addition to these functions they are endowed 

 with a power — only exceptionally exerted, it 

 would seem — of absorbing moisture from the 

 atmosphere, and with the faculty of exhaling 

 watery vapour, or even liquid water, through 

 the stomata or water-pores. This latter power 

 is constantly exercised when the temperature 

 is sufficiently high, and when it is carried to 

 excess the plants commence to wilt. To pre- 

 serve the healthy condition of the plant there 

 must be co-operation between root and leaf, 

 and a proper relative proportion between the 

 quantity of water absorbed by the roots and 

 the quantity exhaled by the leaves. The struc- 

 ture and conformation of the leaves are often 

 profoundly modified, so as to secure the plant 

 from excessive evaporation. This is effected by 

 an increased thickness of the epidermis, as in 

 Cacti, by diminution of the leaf surface exposed 

 to the light and heat of the arid deserts, by hairy 

 coatings, by the storage and accumulation of 

 water in the tissues, and by divers other means 

 which offer most interesting and valuable objects 

 of study. 



In the process of evaporation the stomates, 

 as has been previously stated, play a large part : 

 thus, when the atmosphere is very dry, the 

 "guard-cells" on either side of the stomate are 

 flaccid and in contact so as to close the aperture 

 and prevent the egress of water; but if the air 

 be moist, and the plant well provided with 

 water, then the guard -cells are distended and 

 the gap between them is widened to permit the 

 passage of the superfluous water. 



The Movement of Liquids in Plants. 



It has been shown that root-action and leaf- 

 action are closely associated, and that a due 

 proportion must be observed between the one 

 and the other during the active periods of the 

 plant's life. It is therefore obvious that there 

 must be a direct connection between the root 

 and the leaves, and that this connection must be 

 effected by the stem. The varying anatomical 

 structure of the several parts has been alluded 

 to, and their use now becomes apparent. Some 

 of the parts are adapted for the conveyance of 

 water or other fluids, some for strengthening or 

 for protective purposes, some for storage. The 

 water which rises from the roots passes upwards 

 in spring, under the influence of increasing 

 heat, with great force and rapidity, as may be 

 witnessed when a vine "bleeds". It passes 



