260 



METAMORPHOSIS 



fixes itself and forms a new cell-wall, in the form of an excretion on the outside 

 of the protoplasm. In other cases, also, the cell-wall arises by excretion, but 

 this mode of formation is not universal ; for it has been definitely shown that 

 in some cases the wall arises by transformation of protoplasm, i. e. by a direct 

 alteration of protoplasmic strands. When this method is adopted, carbohydrates 

 must be split off from the protoplasm and ttye nitrogenous remainder be with- 

 drawn, because the wall so formed exhibits the same characters as that formed 

 by excretion. Cell-walls which arise by this solidifying of the protoplasm must 

 have .an extremely complicated composition, and CORRENS (1898) has shown 

 that this is indeed the case. Cell-wall formation by excretion is much more 

 common than by transformation of the protoplasm. 



Cell-wall formation may also be artificially induced. A new wall may 

 appear under certain conditions on the surface of the protoplasts of plasmolysed 

 cells, and, further, isolated protoplasmic particles (e. g. in the Siphoneae) may 

 be induced by mechanical means to form cell-walls. 



The majority of cell- walls have, apart altogether from their mode of origin, 

 the power of growth ; they increase both superficially and in thickness. Surface- 

 growth takes place first, growth in thickness follows later and continues long 

 after surface-growth has ceased. Although these two processes in many cases 



Fig. 40.. Diagram of apical 

 growth in a fungus hypha. 

 After REINHARDT (1802). 



Fig. 50. Stellate parenchyma of Thalia 

 dealoata \ f, in the young state ; //, more fully 

 developed. After ZIMMERMANN (1893). 



take place at the same time and affect each other, we will as far as possible treat 

 of them separately. Let us begin with superficial growth, which is of impor- 

 tance from the point of view of the configuration of the organism, since the shape 

 of the adult cell is determined by the superficial growth of its membrane. We 

 have already made ourselves acquainted with the temporary alterations in form 

 of cells produced by turgor only (stomata, p. 39), and we will recur to these 

 changes elsewhere. 



Let us endeavour, in the first place, to gain some idea of the different 

 types of superficial growth. We know of only a few cases where superficial 

 growth takes place equally on all sides, where an increase takes place without 

 any alteration in form ; as examples we may cite approximately tetrahedral 

 pollen-grains and spores (compare Fig. 54, p. 264) and the cylindrical cells of 

 Hydrodictyon. Generally, only certain parts of the cell-wall grow superficially 

 and these parts may be distributed in various ways on or between parts which 

 show no such growth. Examples of such localized surface growth are to be 

 seen in the simple spherical cells of some Algae (Pleurococcaceae) which are 

 hemispherical in the young state and are hence limited on one side by a flattened 

 cell-membrane. This flat surface alone grows and transforms the hemisphere into 

 a complete sphere.' In many cylindrical cells also, e. g. in the Conjugatae, surface 

 growth is localized, for the cylindrical wall alone grows while the disk-shaped trans- 

 verse walls retain their original dimensions ; the cell-wall increases in extent 

 but not in thickness. In both the examples we have selected the greater part of 

 the cell -wall undergoes surface growth, but there are many cases known where 



