CAUSAL RELATIONSHIPS OF GROWTH AND DEVELOPMENT 27 



callus over a wound, owing to the activity excited in the meristematic cells 

 adjoining the cut surface. These and similar cases are instances of the 

 pronounced and far-reaching effects produced by stimuli of this nature ; 

 for, by the development of dormant buds, reserve food material, hitherto 

 localized in distant parts, may be prepared for transport and gradually 

 removed. Similarly, an injury to the root may produce a perceptible effect 

 upon the most distantly situated stem apices. 



In a well-ordered community every individual is of use and service to 

 the whole, and under conditions which necessitate a rearrangement of 

 the functions of the several members, any given official may be compelled 

 to engage in unaccustomed work and perform duties from which he was 

 previously free. Similarly, in the plant community the activity of every 

 cell and of every organ is subservient to the common weal, and may, when 

 necessary, be modified as already indicated so as to fulfil the changed 

 requirements of the whole. Freed from this communal dependence, an 

 isolated part may exhibit latent powers, which were previously but 

 little, even if at all, employed ; for in all cases the autonomy of the living 

 cell must necessarily be restricted and under control, so long as it is a 

 subordinate member of the whole. 



With progressive development and special adaptation to particular 

 aims and purposes, new powers and properties may be developed or 

 original ones intensified ; while others which were present in embryonic 

 life may be weakened or lost in the adult condition. Thus it is that every 

 cell or organ is not necessarily capable of reproducing the entire plant, 

 even though the power of growth is retained ; and this is, for example, the 

 case with the pollen grain, although it is endowed with marked develop- 

 mental powers. On the other hand, fragments of roots and leaves, consisting 

 of a few or even single cells, may produce buds, and ultimately new and 

 complete plants. In such cases the inherent reconstructive power, doomed 

 under normal conditions to remain for ever dormant, still persisted, although 

 in the intact organ the cells were adult and adapted to perform special 

 functions. The same considerations apparently apply to the individual cell, 

 for although a formation of a new plant from a single isolated cell has not 

 been observed as yet in the higher multicellular plants, this is obviously due 

 to the difficulty of presenting such a cell with a supply of nutriment of ap- 

 propriate quality and quantity. No one can doubt that the fertilized ovum 

 has inherent in it the power of building up the entire organism, although it 

 has not been found possible as yet to reproduce the conditions necessary 

 for its development outside the embryo sac. From such negative results 

 no decisive conclusions can be drawn with regard to what potential powers 

 may actually be inherent, for, in every case, these can only become manifest 

 when certain special and specific external conditions are maintained. 



The suspended development of the unfertilized ovum is an excellent 



