10 POPULAR ILLUSTRATIONS OF 



If we refer back to Fig. 2 (p. 3), we shall observe the plumule is 

 marked out in the embryo plant at g. 



As the root grows by the multiplication of cells, so does the stem 

 increase and rise up above the surface of the soil. As the root 

 derived its primitive nutriment from the sugar of the seed, so does 

 the stem. The stem is destined, however, for different purposes. It 

 has to bear the leaves and branches and fruit of the plant. There- 

 fore, it has to be fashioned with distinct, direct, and unmistakable 

 reference to these objects. The first leaf of the wheat-plant is the 

 cotyledon (d, Fig. 2, p. 3). So soon as the stem appears above 

 the ground, and forms its first true leaf, the use of the cotyledon is 

 finished, and it dies. Every now and then, however, as the wheat- 

 plant rises up above the ground, there is formed a kind of platform 

 from which springs a leaf. This is called a " node," and the spaces 

 between each node are called "internodes." 



What are the exigencies of the wheat-plant ? It has to carry a 

 heavy ear containing the grain, and provision must be made so that 

 it shall bear this weight, and bend with it before the stormy wind 

 without breaking. If the stem were solid it is obvious that it would 

 snap across and break, and the grain would be lost. Therefore we 

 find the form of cylinder is adopted, and the geometrical scholar 

 knows full well that this is the form which a given weight of 

 material will work up into the strongest support. But annual plants 

 have, more or less, succulent, soft stems, and unless that of the 

 wheat-plant were strengthened in a peculiar manner, the stem 

 would not bear its weight. This is done in two ways ; first, by the 

 interposition of the nodes ; and, secondly, by incasing the stem 

 with a coat of flint. I need say nothing about the obvious mode in 

 which the nodes strengthen the stem. The flinty coat requires a 

 word or two of notice. If a piece of wheat-straw be boiled in nitric 

 acid the vegetable matter will be dissolved, and a perfect cast of the 

 straw in pure transparent flint may be obtained, as shown in Fig. 4, 

 page 8. 



But how is this ? Have I not, in describing the roots, shown that 

 all matters taken up by them must be in solution, and yet I now 

 say that the straw contains a cast of flint, which is insoluble in that 

 which will dissolve almost everything nitric acid ? How is this to 

 be explained ? Just in this wise. The soil contains flint in a state 

 of minute subdivision, but still insoluble. When the elementary 

 principle, silicon, is acted upon by the oxygen of the atmospheric 

 air, which is allowed by our system of cultivation to get into the 

 soil, it becomes oxidised, and "silica," or "silicic acid" the terms 

 being synonymous, is formed. The silica exists nearly in a state of 

 purity in rock crystal, common quartz, flint, chalcedony, &c., and 



