66 



THE AGRICULTURAL NEWS. 



Mauch 1. 19K 



germinate in the fruit, although they are surrounded or receiving a last dehydration and being deposited as 

 by an abundance of moisture. permanent tissue. 



Common observation teaches that the sprouts 

 from a .seed always contain a much greater jiroportion 

 of water than the remainder of the seed. In this very 

 early stage of the seedling, there is little provision for 

 the direct absorption of water, and the latter must be 

 formed in some way in the developing embryo. The 

 fact is that the newly- formed tissues are respiring 

 actively, while no respiration is taking place in the old 

 parts of the seed; so that the former are more succulent 

 simply because of the large amount of metabolic water 

 arising from their rapid respiration. 



In order to form a mental picture of what takes 

 place through the action of light in green leaves, :ind 

 in further changes brought about in the ceils of the 

 plant.it must be realized that the formation of different 

 kinds of carbohydrates is all-important in these matters. 

 These carbohydrates may be regarded, for the puiposes 

 of the ])resent discussion, as substances that differ from 

 one another in the proportion of the elements of water 

 that they contain; that is, in their state of hydration. 

 Among carbohydrates possessing the highest hydration 

 are the sugars dextrose and levulose; starch and cellu- 

 lose (the latter the stuff of which cell walla are chiefly 

 made) represent the lowest state of hydration among 

 these bodies, while there are many carbohydrates 

 intermediate between these, in this respect. The first 

 stable carbohydrate to be formed in green leaves is 

 Starch, which is hydrolysed by enzymes to a soluble 

 carbohydrate, usually de.xtrose, and this travels through 

 the plant from cell to cell, by osmosis. Intramolecular 

 respiration within the cells causes part of the dextrose 

 to be oxidized to carbon dioxide and water, and the 

 energy thus liberated brings about the dehydration of 

 other parts of it to form bodies such as cellulose, starch 

 and cane-sugar. It is in this way that the starch stored 

 in plants, and the cellulose of the cell walls, are formed: 

 and in the meantime the water aiising from the intra- 

 molecular respiration travels (together with water from 

 the roots) away from the cells to the leaves, carrying 

 •with it the waste products of metabolism. It is 

 a peculiar property of plants, in distinction to animals, 

 that they can reconvert these waste products into 

 useful nutrients, this work being one of the functions 

 of the leaves. A matter of interest is that the same 

 group of atoms may complete the cycle several times, 

 carrying combined water from the leaves to the grow- 

 ing cells, and linally becming oxidized in respiration 



Fruits, again, afford an example of the usefulness 

 of metabolic water. The final ripening of most fruits 

 will take place after they have been removed from the 

 plant, and this indicates that the water required for 

 the increased succulence attendant oa maturity is not 

 derived from the plant on which they grow. Respira- 

 tion occurs in ripening fruits, so that water is formed 

 in them, and their juiciness also becomes greater 

 because the proportion of soluble substances increases 

 as they mature. It may be proved that respiration is 

 necessary in the process by withholding oxygen, when 

 il1s^pad of ripf;ning normally, the fruits will undergo 

 changes similar to those that take place in ensilage. 



It has been !^hown that animal cells respire in 

 a manner similar to those of plants, with the same 

 general results. The metabolic water from this respira- 

 tion weakens the soliition of the cell contents, so that 

 food materials pass from the stronger solution in the 

 blood, through the cell walls, by osmosis. This is the 

 way in which a constant flow of nutrient toward the 

 the tissues is insured, so that the material used up 

 in respiration and growth may be replaced. The 

 great difference between plants and animals, as has 

 been indicated, is that animals cannot use the waste 

 products for building ttp fresh nutrients: these must be 

 excreted quickly, or the tissues will be poisoned. It is 

 of interest to consider how this excretion takes place in 

 different kinds of animals. 



The form in which nitrogenous w.xste products from 

 most animals are excreted is a soluble body called 

 urea, whose removal requires the use of a large quan- 

 tity of water. Insects, birds and reptiles, however, get 

 rid of the waste in the form of insoluble salts of uric 

 acid, which pass out of the body with a miLimum loss 

 of water. As uric acid contains a smaller proportion of 

 hydrogen than any other nitrogenous compound 

 excreted by animals, this fact in itself hel])s to lessen 

 the loss in the ease of the latter animals. This means 

 that insects, birds and reptiles lose little of their 

 metabolic water in the process of excretion: while if 

 ordinary- animals are fed on a diet containing minimum 

 quantities of nitrogenous substances (proteids), less 

 metabolic water will be required for that process. The 

 practical results of this may be considered, taking the 

 latter case first. 



The matter possesses its importance, in the higher 

 animals, in the case of those which hibernate. In this 



