What is the principal constituent of the potato root? Of the carrot and turnip? What part of the plant contains usually the most nutriment?] Let us now examine plants with a view to learning the _location_ of the various plants. The stem or trunk of the plant or tree consists almost entirely of _woody fibre_; this also forms a large portion of the other parts except the seeds, and, in some instances, the roots. The roots of the potato contain large quantities of _starch_. Other roots such as the _carrot_ and _turnip_ contain _pectic acid_,[J] a nutritious substance resembling starch. It is in the _seed_ however that the more nutritive portions of most plants exist, and here they maintain certain relative positions which it is well to understand, and which can be best explained by reference to the following figures, as described by Prof. Johnston:-- [Illustration: Fig. 1.] "Thus _a_ shows the position of the oil in the outer part of the seed--it exists in minute drops, inclosed in six-sided cells, which consists chiefly of gluten; _b_, the position and comparative quantity of the starch, which in the heart of the seed is mixed with only a small proportion of gluten; _c_, the germ or chit which contains much gluten."[K] [Is the composition of the inorganic matter of different parts of the plant the same, or different? What is the difference between the ash of the straw and that of the grain of wheat?] The location of the _inorganic_ part of plants is one of much interest, and shows the adaptation of each part to its particular use. Take a wheat plant, for instance--the stalk, the leaf, and the grain, show in their ashes, important difference of composition. The stalk or straw contains three or four times as large a proportion of ash as the grain, and a no less remarkable difference of composition may be noticed in the ashes of the two parts. In that of the straw, we find a large proportion of silica and scarcely any phosphoric acid, while in that of the grain there is scarcely a trace of silica, although phosphoric acid constitutes more than one half of the entire weight. The leaves contain a considerable quantity of lime. [What is the reason for this difference? In what part of the grain does phosphoric acid exist most largely?]