CELL-ORGANS 21 



with its complex morphological character: a great number of differenl 

 substances enter into the composition of protoplasm, prominent among 

 them being the proteins, which are themselves, of course, the most 

 complex carbon compounds known to the chemist. 7 



Since the correlation of structure with function is always evident, 

 as regards both the gross anatomy and the histology of the plant-body, 

 a similar connection between the morphological features of protoplasm 

 and the functions of the individual protoplast would doubtless be 

 demonstrable, were it not that most of the structures concerned are 

 of ultramicroscopic dimensions. It is, nevertheless, possible to recognise 

 within the living substance a number of well differentiated cell-organs 

 which are entrusted with definite functions. 



The arrangement of the protoplasm within the cell and the disposition 

 of the plasmatic organs are not always the same, but depend at any 

 given moment partly upon the stage of development to which the 

 protoplast has attained and partly upon the nature of its special 

 activities. Moreover, external stimuli may affect both the general 

 disposition of the protoplasm and the arrangement of the cell-organs. 

 In the case of the embryonic cells which compose the growing-points 

 of a Higher Plant, it is usual for the cell-cavity to be completely filled 

 with protoplasm. At the centre of such a cell will be found the most 

 important of all the cell-organs, namely, the nucleus. The centrosome 

 is a minute organ, which is of general occurrence in animal cells, but 

 which among plants has been identified with certainty only in some of 

 the Lower Cryptogams ; where it is present, it is always closely associated 

 with the nucleus. Grouped around the nucleus are several colourless 

 or pale-green corpuscles of moderate size, the chromatophores. All the 

 aforesaid organs lie embedded in the general cell-plasma or cytoplasm, 

 which is bounded externally, that is to say towards the cell-wall, by 

 the ectoplast (Hautschicht). In the adult cell the conditions are 

 altered, owing to the appearance within the cytoplasm of cavities filled 

 with cell- sap, the so-called vacuoles. The vacuoles may ultimately be 

 replaced by a single large sap-cavity : in this case the protoplast, with 

 its attendant organs, assumes the form of a peripheral layer of varying 

 thickness (the "primordial utricle" of v. Mohl), which adheres to the 

 inner surface of the cell-wall. Not invariably, however, does the 

 whole of the cytoplasm shift on to the walls after the formation of a 

 sap-cavity ; very often the latter may be seen to be traversed in various 

 directions by cytoplasmic strands and filaments which sometimes connect 

 a central mass of plasma, in which the nucleus is suspended, with the 

 peripheral layer (Fig. 1 ). 



The cytoplasm apart from the ectoplast, which is always stationary 

 very frequently displays a more or less energetic streaming movement 



