20 The Living Plant 



reader should experiment also upon its instability in sunlight, a fact 

 of importance as will later be proven; this he may do by dividing 

 his solution into two portions, of which he puts one in bright 

 sunlight and awaits its changes of color, while he places the 

 other in darkness for comparison. Incidentally, too, this experi- 

 ment will show an important fact about the color of leaves apart 

 from their coloring matters, for, when the action of the alcohol 

 is complete, the leaves appear a soft creamy white. This, in 

 fact, is the natural color of all living plant tissues when no special 

 coloring material is present. 



We must, however, pursue a bit farther the study of the chloro- 

 phyll substance, partly because of its importance, and partly 

 because the study will lead the reader to an acquaintance with 

 other matters which he should learn very early in his botanical 

 studies. To the naked eye alone, no matter how closely applied, 

 the chlorophyll seems to color uniformly the whole of the leaf, 

 which, except for the veins, looks homogeneous in texture. But if 

 we call to aid that wonderful instrument by which the range of the 

 eye into the minute is increased a full thousandfold, that first 

 and greatest tool of the biologist, the microscope, and place 

 under its lenses a very thin section or slice cut right through some 

 green leaf from surface to surface, then a very different idea of 

 leaf structure is presented to the observer, as the accompanying 

 picture attests (figure 2). And with this picture of an actual leaf, 

 the reader should compare the generalized or conventionalized 

 section represented in figure B on Plate I. Clearly, the interior 

 of the leaf is not homogenous, but partitioned into a great many 

 little compartments, with empty spaces here and there inter- 

 spersed. These compartments are called cells, a word of vast 

 importance in Biology, because not only the leaf, but all parts 

 of all plants, and all parts of all animals, are composed of them. 

 These cells differ greatly in details of structure according to their 

 function, but are always compartments of some sort; and the 

 reader should as promptly as possible incorporate this idea of 



