METABOLISM 



m 



living plant localized in definite places, and cannot, under such circumstances, 

 react in any way on each other. A glance through a microscope shows us 

 how extraordinarily complicated in structure a plant really is. Every single 

 organ is seen to be composed of innumerable units which we term cells, while, 

 on the other hand, the entire body of a microscopic alga may consist of but 

 one cell, similar to some one of those which occur in a higher plant. If we 

 collect a number of these unicellular Algae and submit them, in mass, to chemical 

 analysis, we shall obtain results in no respect differing from those which we 

 meet with in a chemical analysis of the most highly differentiated parts of plants 

 of much higher grade. It is manifestly of the highest importance that we 

 should make ourselves acquainted with the constituent parts of a cell as revealed 

 by the microscope, and ascertain, if possible, how the different substances 

 which we have classified above are distributed in the cell. For this purpose we 

 must employ not only the ordinary methods of chemical analysis, but also the 

 so-called ' microchemical reactions ', on whose further 

 development the extension of our knowledge in this 

 direction so largely depends. Even now, however, it is 

 possible to determine in situ under the microscope 

 a considerable number of chemical compounds. To 

 enumerate all the microchemical reagents and their 

 reactions would occupy far too much space ; we must, 

 therefore, content ourselves with a resume of the most 

 important results arrived at ; in doing so, however, it 

 will be impossible to avoid trenching on certain ques- 

 tions which are morphological rather than chemical. 



We may select as a subject for study a cell of the 

 freshwater alga, Dmparnaldia glomerata, illustrated at 

 Fig. i. This cell is cylindrical in form, and in it may 

 be distinguished three primary constituents : (i) the 

 cell-wall (m), which forms a hollow cylinder enclosing 

 cell contents ; (2) a soft, viscid substance, the protoplasm 

 (pi), covering and closely applied to the inner surface 

 of the cell-wall, and forming, like it, a closed sac ; (3) the 

 cell-sap (the vacuole, v), occupying the remainder of the 

 space. Although no further structural differentiations 

 are obvious in the cell-wall or cell-sap, these are by no 

 means absent from the protoplasm. In the first place, 

 we notice an annular green band with ragged edges, 

 the chloroplast (ch), a spherical body known as the 

 nucleus (n), and finally the cytoplasm, i.e. the remainder 

 of the protoplasm, a colourless, translucent mass, whose exact nature it is 

 extremely difficult to determine, and in which the chloroplast and the nucleus 

 lie imbedded. 



Structures of similar or nearly similar character are met with in the majority 

 of vegetable cells, the difference lying for the most part in the form of the 

 chloroplast. It is only rarely that that body has the characters seen in Dra- 

 parnaldia, it is generally much simpler in structure ; each cell may contain 

 many chloroplasts, or these may be wanting altogether. The parts of the cell 

 above enumerated are of very different value, for the functions fulfilled by the 

 protoplasm are of much greater moment than those carried out by the cell-wall 

 and cell-sap, the latter being really products of the former. Indeed, cells 

 are not unknown which consist either for a time or during their entire life 

 of protoplasm only. Protoplasm may be briefly defined as the living substance 

 of the plant (and of the animal also), for it is only in such parts as contain 

 protoplasm that we encounter those changes which we recognize as characteristic 



Fig. i. Cell of Drapar- 

 naldia glomerata. m^ cell- 

 wall ; //, protoplasm ; ch, 

 chloroplast ; , nucleus ; z>, 

 vacuole. (Magnified about 

 500.) 



