MORPHOLOGY 



49 



chromatophores. In the protoplasm of the lowest plants, the Fission 

 plants or the Schizophytes, the internal differentiation does not seem 

 to have progressed to the same extent as in 

 the more highly organised plants. 



The protoplasm of animal cells, on the 

 other hand, is devoid of chromatophores. 

 While animal cells usually remain continuously 

 filled with protoplasm, vegetable cells soon 

 form large SAP cavities. It is only the 

 embryonic cells of plants that are entirely 

 filled with protoplasm, as the cells, for ex- 

 ample, of an ovule or of a growing point ; 

 they afterwards become larger and contain 

 proportionally less protoplasm. This can be 

 seen in any longitudinal section through a 

 stem apex. At a short distance from the 

 growing point the enlarged cells have already 

 begun to show cavities or vacuoles (v in A, 

 Fig. 51) in their cytoplasm. These are filled 

 with a watery fluid, the cell sap. The cells 

 continue to increase in size, and usually soon 

 reach a condition in which their whole central 

 portion is filled by a single, large sap cavity 

 (v in B, Fig. 51). This is almost always the 

 case when the increase in the size of the cell 

 is considerable. The cytoplasm then forms 

 only a thin layer lining the cell wall, while 

 the nucleus takes a parietal position in the 

 periphera). cytoplasmic layer. At other times, 

 however, the sap cavity of a fully-developed 

 cell may be traversed by bands and threads 

 of cytoplasm; and in that case the nucleus 

 is suspended in the centre of the cell. But 

 whatever position the nucleus may occupy, it 

 is always embedded in cytoplasm ; and there 

 is always an unbroken peripheral layer of 

 cytoplasm lining the cell wall. 



This cytoplasmic peripheral layer is in con- 

 tact with the cell wall at all points, and, so long 

 as the cell remains living, it continues in that 

 condition. In old cells, however, this cyto- 

 plasmic layer frequently becomes so thin as to 

 escape direct observation, and is not perceptible 



until some dehydrating reagent, which causes it to recede from the wall, 

 has been employed. Such a thin cytoplasmic peripheral layer has been 

 described by Hugo v. Mohl under the name of primordial utricle. 



t"J 



Fig. 51. —Two cells taken at 

 different distances from the 

 growing point of a phanero- 

 gamic shoot, fc, Nucleus ; 

 cy, cytoplasm ; v, vacuoles, 

 represented in B by the sap 

 cavity. (Somewhat diagram- 

 matic, x circa 500.) 



