THE DIFFERENTIATION OF CELLS AND OF ORGANS 139 



Instances have already been given to show that different organisms 

 vary widely as regards their properties, powers, and needs, that the shape 

 may change in correspondence with the external conditions, and that 

 alterations in the latter may be essential for the completion of the life-cycle. 

 In addition many new or unusual growths are only formed under the action 

 of special stimuli, which may also be required for the development of 

 sporangia, flowers, and other organs. 



In most cases the germ develops normally under constant and uniform 

 external conditions, the subsequent differentiation resulting from its in- 

 herited properties. Such development is automorphic, but in some cases 

 a physiologically radial germ-cell has its main axis of growth determined 

 by external stimuli, and organisms may exist which are unable to develop 

 in the absence of the appropriate orienting stimulus. 



When tissue-differentiation and division of labour begin, it requires to 

 be determined whether the different meristems diverge from one another in 

 their specific characters, or whether they remain similar and equipotential. 

 The latter is usually the case, so that the dissimilar differentiation of 

 segment-cells is determined by the action of external factors, or of 

 internal ones, such as the influences arising from the preformed parts. The 

 outermost layer of meristem unavoidably develops into the epidermis, and 

 if the outer layers are removed, the most external layer of living meristem 

 forms a new epidermis. Similarly a further injury may cause cells which 

 would have developed into vascular or parenchymatous elements, according 

 to their position, to form callus-tissue and a layer of cork. 



It has not yet been found possible to raise a flowering plant from an artificially 

 isolated meristem cell, but in many plants single cells or groups of cells may form 

 buds on root, stem, or leaf, and these may develop into new plants, showing that 

 the original cells were similar in character and possessed of the same productive 

 powers. This vegetative reproduction is favoured by the disturbance or interrup- 

 tion of the correlative determining influences exerted by associated parts, and 

 the mode of removal or plane of incision may determine whether a particular 

 group of cells develops into a root or shoot *. Even when the new formations are 

 produced from cells which under normal conditions had ceased to grow, the 

 experiments are completely satisfactory, for the segment-cells could not possess 

 the general powers requisite for the formation of a new plant unless these powers 

 were inherent in the original cells. 



So long as the determining factors remain constant, the character of 

 development will be unaltered, and the fact that the formative activity of the 

 stem- and root-apices is comparatively unaffected by a variety of agencies, 

 simply shows that these agencies exert no pronounced influence upon the 



1 Vochting, Organbildung im Pflanzenreich, 1878, I, p. 240; 1884, II, p. 36. 



