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 ma- 

 terial, but that we also recognise the presence within the 

 blood and within the nervous system of the enormous 

 mass of minute centres of action. 



In conclusion, I have still some preparations to ex- 

 plain, 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 begin- 

 ning 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 con- 

 sists 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 dis- 

 tinguished 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). 6. Larger, four-sided cells of the same kind from the cam- 

 bium ; the real cell (primordial utricle), shrunken and wrinkled, within the capsule. 

 c. Cells with starch-granules lying within the primordial utricle. 



