8 



CONTENTS OP CELLS. 



Nucleoli are not always present. They are either vesicles and 

 granules contained in the nucleus, or minute cavities in its substance. 

 The latter view is supported by Barry, who holds that a peculiar sub- 

 stance, called hyaline (JAoj, glass), is developed there, which, accord- 

 ing to him, is the origin of the nucleus. The nucleus is situated at 

 different parts of the cell. It is either free in its cavity, or connected 

 with its wall by mucilaginous threads, or imbedded in the substance of 

 the membrane. The addition of acetic acid often renders the nucleus 

 distinct. 



17. Starchy matter is found in cells, which constitute the tissue 

 called, by Morren, Perenchyma (vi\^a. y a sac.) Starch exists in the 

 form of granules, which are minute cells, (perhaps nuclei, as Mulder 

 states,) in which nutritious matter is stored up. This matter may be 

 deposited in such a way as to give the appearance of stria? surrounding 

 a point or hilum, which is considered as an opening into the cell. 

 The grains of starch are well seen in the cells of the potato (fig. 34). 

 In wheat (fig. 35), and in maize (fig. 36), the form of the granules, 



and the successive layers of deposit, are also seen. The grains in the 

 stem of Nuphar luteum show the centripetal formation, that is, the 

 increase by layers deposited within each other. The addition of iodine 

 causes the grains of starch to assume a blue colour, and marks the 

 difference between them and the walls of the cell containing them. 

 18. Crystals are found in the interior of cells. They probably 

 owe their origin to the union between the acids pro- 

 duced or taken up by plants, as oxalic, phosphoric, 

 malic and carbonic, and the alkaline matter, as lime 

 and potash, absorbed from the soil and circulating in the 

 sap. The crystals usually lie loose in the cells (figs. 37, 

 38) ; but, according to Pay en, they are sometimes found 

 in a distinct tissue, and suspended from the wall of a 

 large cell (fig. 39) filling what some have supposed to 

 be the base of an undeveloped hair. The crystals are 

 of different sizes and forms. Occasionally, a single large 

 crystal nearly fills a cell, but in general there are numerous crys- 

 tals united together. Sometimes the crystals radiate from a common 

 point (figs. 40, 41), and form a conglomerate mass; at other times they 



Fig. 34. Cell of Potato, containing striated starch grains. 

 Fig. 35. Grains of starch of Wheat. Fig. 36. Grains of starch of Maize. 



Fig. 37. Cellular tissue of Arum maculatum. c, Cells containing chlorophylle. r r, Raphidian 

 cells. 



