SECT. I 



MORPHOLOGY 



53 





ecach of the cells there will be clearly distinguishable a round body 

 (k), which fills up the greater part of the cell cavity. This body is 

 the cell NUCLEUS. If sections, made in different directions through 

 the vegetative cone, be compared with one another, it will be seen 

 that its component cells are nearly cubical or tabular, while the 

 nuclei are more or less spherical or disc-shaped. The finely granular 

 substance (aj) filling in the space between the nucleus (l) and the cell 

 wall (m) is the cell plasm or CYTOPLASM. In the cytoplasm there 

 are to be found, about the nucleus, a number of colourless and highly 

 refractive bodies : these are the pigment-bearers or chromatophores 

 (ch). Nucleus, cytoplasm, and chromatophores, constitute the 



ELEMENTS OF THE LIVING BODY OF A TYPICAL VEGETABLE CELL. To 



designate all these collectively, it is customary to use the term proto- 

 plasm, which is then to be understood 

 as including all the living constituents 

 of the cell-body or protoplast. 



In many animal cells modern in- 

 vestigations have revealed, in addition 

 to the constituents of the protoplasm 

 just mentioned, a small structure situ- 

 ated close to the nucleus, which has 

 been termed the centrosome, centri- 

 OLE, or attraction-sphere. Similar 

 structures have been demonstrated in 

 some Thallophyta and Bryophyta (Fig. 

 56 c;); they appear to be wanting in 

 the cells of the higher Cryptogams 

 and the Phanerogams. 



The nucleus and cytoplasm are the 

 two most essential constituents of the 

 cell, and its vital functions depend on 

 the interaction between them. In the lowest plants (Cyanophyceae 

 and Bacteria) such a division of labour in the protoplasm is not 

 certainly proved, the existence of a nucleus being still a matter of 

 dispute (^-). Chromatophores are wanting in the Bacteria and Fungi, 

 as in all animal cells. 



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 example, of an embryo 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. 57) in their 

 cytoplasm. These are filled with a watery fluid, the cell sap. The 

 cells continue to increase in size, and usually soon attain a condition 



Fio. 56.— A nucleus of a cell of the young 

 plant of Fiicus serratus, a Brown Sea- 

 weed, cy, The surrounding cytoplasm ; 

 k, the nucleus ; kw, nuclear membrane ; 

 n, nucleolus ; c, centrosome ; ch, chro- 

 matopliores. (x 1000.) 



