46 BOTANY 



that in the growing cell-wall there is more or less living protoplasm 

 concerned in the laying down of new cellulose molecules. 



Vacuoles In very young cells the cytoplasm as a rule fills the cell 

 completely, but as the cells increase in volume there is not a cor- 

 responding growth of the protoplast, which in consequence develops 

 cavities within it tilled with watery fluid, or cell-sap. In old plant- 

 cells there is generally a single large central vacuole, and the cyto- 

 plasm is reduced to a thin membrane closely appressed to the cell-wall 

 by the pressure of the fluid contained within the central vacuole. 

 Not infrequently, as in the cells of many plant-hairs (Fig. 29), the 

 large vacuole is traversed by threads in bands of cytoplasm in which 

 active streaming can usually be seen. 



In the free-swimming zoospores of many Algae, and in the vegeta- 

 tive cells of the Volvocaceae, there are found small vacuoles which 

 contract and expand rhythmically, and are comparable to the con- 

 tractile vacuoles found in many Infusoria. The fluid within the 

 'vacuoles is not pure water, but contains various substances in solu- 

 tion, which may become precipitated. Such precipitates are the 

 characteristic calcium crystals met with in many plants. 



The Nucleus. In all typical cells there is a definite Nucleus, which 

 has been shown to be a structure quite distinct from the cytoplasm. 

 In all cases the nucleus arises by division of a preexisting nucleus. 

 In the living cell the nucleus usually presents a more or less granu- 

 lar appearance and contains one or more nucleoli. The membrane 

 bounding the nucleus is analogous to that about the vacuoles, and 

 like it belongs to the cytoplasm. 



Plastids (Chromatophores) In most plant-cells there can usually 



be found characteristic bodies embedded in the cytoplasm, and which, 

 like the nucleus, can never be formed de novo in the cytoplasm. 

 These are the Plastids, or Chromatophores, and include the green 

 corpuscles, or chlorophyll-granules (Chloroplasts), which give the 

 characteristic green color to plants. 



Centrosomes In the cells of some Brown Algae (Fig. 31), and also 

 in a few Liverworts, e.g. Pellia, structures known as Centrosomes 

 have been detected, which resemble the corresponding structures in 

 the cells of animals. These are minute granules lying close to the 

 nucleus, and sometimes showing a marked radiation in the surround- 

 ing cytoplasm. It has been assumed that these bodies are of much 

 importance as centres of energy, and are constant structures like the 

 nucleus; but the results of later study tend to prove Ihat they are 

 absent from the cells of the higher plants, and are probably of nmch 

 less importance than was formerly supposed. 



Multinucleate Cells While most plant-cells possess but a single 

 nucleus, there are many examples of protoplasts provided with 

 several or many nuclei. Such are the giant cells of the Water-net 



