58 THE ELEMENTARY STRUCTURE OF PLANTS. 



state, it receives the name of protoplasm. Besides lining the walls 

 of living cells and foi'ming the nucleus, it is also a component of one 

 of the most important vegetable products, viz. 



92. Cllloroplljil, or, as the name denotes, Leaf-green, the substance 

 which gives the universal green color to the leaves and herbage. 

 This is formed principally in parts exposed to the light, such as the 

 green bark, and especially the leaves. It generally occurs in the 

 form of minute soft granules, either separate or in clusters, which 

 lie free in the cells (Fig. 71), or adhere loosely to their sides. In 

 some common Confervae the chlorophyll takes the form of rows of 

 granules, or of continuous bands, often spiral in form. The exact 

 composition of chlorophyll is still unkno^vn. The green coloring 

 matter makes only a small part of the bulk of the grains. It may 

 be dissolved out by alcohol or ether, leaving a coloi'less mass, which, 

 as it is turned yellow by iodine, evidently contains nitrogen. The 

 green matter is found to consist partly of wax, and jjartly of a pecu- 

 har quaternary substance allied to indigo. 



93. Earthy Incrustations. As the roots naturally take in some 

 earthy matters, dissolved in the water they absorb from the soil, 

 these necessarily accumulate in the cells of the plant. The siliceous 

 and calcareous matters, being very sparingly soluble, are usually 

 deposited on the walls of the cells as an incrusting lining, or else 

 are incorporated into its substance along with the organic thickening 

 deposit (41). This earthy part of vegetable fabric may be brought 

 to view by carefully burning a piece of a leaf or any other organ, — 

 which decomposes and drives off all the vegetable matter, — and then 

 examining the ashes by the microscope. These are mineral matter, 

 and when undisturbed they will be found to have copied the shape 

 and all the minute markings of the cells, like casts. In the Diato- 

 macea>, — a family of microscopic and ambiguous plants of the sim- 

 plest structure, — a great part of the thickness of the cell-wall is 

 silex, and consequently indestructible by decay. So that the forms 

 of these minute organisms are preserved indefinitely, after the de- 

 composition of the organic structure ; their silicious remains accu- 

 mulating at the bottom of the water in which they Hved, to such 

 extent as to produce immense strata in many places, their forms and 

 markings so perfectly preserved for ages that the species may be 

 nearly as well characterized from these casts as from living indi- 

 viduals. Earthy matters also occur in the cells of plants in the 

 form of microscopic 



