76 Vcffetable Phijsiologi/. 



each still entire, inclosina: its mass of starch grains, and looking, 

 when highly magnified, like a bagiuU of oyster-shells. 



The easiest way, however, to obtain a clear idea of the Atture 

 of these cells of plants, is to examine microscopic plants, lox the 

 size of the cell does not diminish in equal proportion to the size 

 of the smaller plants ; these are composed of fewer cells, and we 

 can descend so far that the number is reduced to the lowest point, 

 so that in the smallest and simplest plants we aie acquainted 

 with, the whole individual plant consists merely of a single 

 little bag, or cell, like one of those we see in such numbers in a 

 slice of the substance of an ordinary plant. 



Most persons must have noticed the green powder which 

 covers the bark of trees, wooden palings, damp walls, &c., look- 

 ing like a mere stain. Its green colour indicates that it is of 

 vegetable nature, and it is well known to botanists as depending 

 upon the presence of countless millions of specimens of a very 

 curious and interesting plant, each single one of which consists 

 simply of a membranous bag of globular form, l-6000th of an 

 inch in diameter, filled with liquid containing green colouring 

 matter. 



The history of this plant is very instructive as regards the 

 nature and mode of growth of vegetable structure, and we shall 

 therefore describe the most important features of it. 



When a small quantity of the green substance is examined by 

 a \ov{ magnifying power it appears to con- 

 '^' * sist of fine grains ; but if we use a high 



power we find that each grain consists of 

 a colourless bag of membrane, like a little 

 bladder, and that it owes its colour to 

 green substance lying in the thick liquid 

 contents, which may be squeezed out by 

 pressure. If acids are applied, the con- 

 tents are seen to contract and become more 

 solid ; they then lie as a little mass in the 

 centre of the bag, the colourless character 

 Cew^ of Protococcm vivid!, in ^^ . wliicli is thus more clearly seen. These 

 diifeient stages and lovms, little bag's or cells cxactly represent the 



magnified 60U diameters. . ^. . ri-iiii c 



microscopic elements, oi which all the soit 

 green parts of the higher plants are composed. In a slice 

 through a leaf of the bay-laurel, for instance, we see that the 

 spongy texture between the skins of the upper and lower sur- 

 faces is composed of innumerable little colourless bags or cells 

 of exactly the same kind, which owe their colour, in like manner, 

 to green substance contained in the liquid with which tlie-v are 

 filled. 



Since the large structures of the higher plants are formed of 



