Structure of Plants. 73 



of the most important of the aids to practical advancement in 

 almost every branch of human industry. Physiology has vindi- 

 cated its position in connexion with the art of medicine, and is 

 daily offering new material for the still more important art of 

 preserving health — hygiene. But in reference to agriculture its 

 importance is not yet thoroughly recognised : nor could it be so 

 while few of the general principles were clearly understood. 

 These waited for the aid of chemistry. Armed with the results 

 furnislied by the sister science, and the means supplied l)y the 

 great improvements made in tlie optical part of microscoj)es 

 within the last twenty years, much may now l)e expected from 

 physiology, on the laws of which indeed (the laws of life) f on- 

 sciouslv, or at present more frequently unconsciously, the whole 

 art of cultivation, the rearing of plants and animals mainly 

 rests. 



We have said above that plants are com])osed of minute but 

 distinctly characterised parts which are called elemeutan/ organs. 

 All vegetable structures are made up of these, and increase in 

 mass by their multiplication and expansion ; all vegetable pro- 

 ducts are elaborated in the interior or deposited in the substance 

 of these elementary organs. The knowledge of these elementary 

 "atoms" then must constitute the groundwork of all knowledge 

 of vegetable life. The examination of the essential general 

 characters of vecjetahle cells, and their modes of multiplica- 

 tion, nmst therefore form the first step in all physiological 

 inquiries. 



If we squeeze a leaf, or any other soft part of a plant, between 

 the fingers, we see liquid exude, showing that the substance is 

 not solid, but of the consistence which is commonly called .*.y;o?//7y. 

 But we should mistake if we imagined that the texture is similar 

 to that of sponge. Sponge is composed of delicate liorny 

 threads interwoven and netted together, and holds liquid in 

 the interspaces between these threads just in the same way as 

 a bundle of tow would do, or as the wick of a lamp soaks up 

 the oil. 



The substance of vegetables is very different from this, and 

 the liquids tiiey contain are not merely diffused through a porous 

 texture, but are contained in closed cases, so that they do not 

 escape unless the parts in which they lie are cut or bruised. li 

 we cut an extremely thin slice of the substance of a leaf and 

 examine this under the microscope, we find that the sponiry struc- 

 ture is composed of a vast number of little l)ags filled with 

 liijuid, somewhat loosely j)a(ked together in the inside ol the 

 leaf, and we find air and not liquid in the interspaces between 

 these bags. 



I'hese little bags are more easilv seen in slices of the soft 



