GROWTH OF PLANTS.
45
ignored, that we no longer content ourselves with merely regarding the nerves as so many wholes, as a simple, indivisible apparatus, or the blood as a merely fluid material, but that we also recognise the presence within the blood and within the nervous system of the enormous mass of minute centres of action.
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In conclusion, I have still some preparations to explain, and will begin with a very common object (Fig. 7). It has been taken from the tuber of a potato, at a spot where you can view in its pefection the structure of a vegetable cell, where the tuber, namely, is beginning to put forth a new shoot, and there is, consequently, a probability of young cells being found, at least, if we suppose that all growth consists in the development of new cells. In the interior of the tuber all the cells are, as is well known, stuffed full with granules of starch ; in the young shoot, on the other hand, the starch is used up, in proportion to the growth, and the cell is again exhibited in its more simple form. In a transverse section of a young sprout near its exit from the tuber, about four different layers may be distinguished — the cortical layer, next a layer of larger, then a layer of smaller, cells, and lastly, quite on the inside, a second layer of larger cells. Here we see nothing but regular structures ; thick capsules of hex-
Fig. 7. From the cortical layer of a tuber of solanum tuberosum, after treatment with iodine and sulphuric acid. a. Flat cortical cells, surrounded by their capsule (cell-wall, membrane). b. Larger, four-sided cells of the same kind from the cambium; the real cell (primordial utricle), shrunken and wrinkled, within the capsule. c. Cells with starch-granules lying within the primordial utricle.
