i8o THE CAUSES OF SPECIFIC SHAPE 



surface-tension due to the action of the plasmatic membrane or the excretion 

 of metabolic products. Life and growth are in fact only possible when the 

 organism is able to convert the energy and materials derived from without 

 to its own uses. Even in individual cells an inherent polarity or a localized 

 change of whatsoever kind may cause constant external conditions to produce 

 a localized reaction. 



The degree to which the external world is used in the manner 

 indicated as a source of orienting stimuli must be separately determined 

 in each case. During ontogenesis such stimuli are apparently often used, 

 although not always in a readily perceptible manner, and it is even possible 

 that in some cases a sense of perception may be active. The directive 

 action of the external world is not, however, a general essential, for the 

 progressive changes involved in development may be automatically 

 produced in regular sequence, and in fact appear mainly or entirely to 

 provide the stimuli to further growth in a definite manner and direction. 



Cellular and nuclear division result from internal causes, and it is 

 impossible to say precisely why they occur when a certain definite size 

 has been attained. That a disturbance of the ratio between the bulk 

 of the cell and its absorbing surface is not essential to produce division, 

 is shown by the fact that the cylindrical cells of Spirogyra^ while retaining 

 the same diameter, divide after they have attained a certain length. It is, 

 however, possible that changes of the ratio between bulk and surface may 

 frequently act as exciting or directive stimuli, and in large organisms the 

 structure and tissue-differentiation is such as to enable the innermost cells 

 to be adequately supplied with oxygen and other food-materials. 



Cell-division is a physiological process, and not the direct physical 

 result of surface-tension forces, even although these may be utilized by the 

 protoplast to produce the required results. A living protoplasmic thread of 

 variable thickness, for example, readily resists the surface-tension forces which 

 would cause a free thread of liquid to break up into drops. It is indeed 

 doubtful whether the division even of a naked spherical or ellipsoidal ovum 

 is ever a purely physical process. 



The above considerations apply not only to isolated cells, but also to 

 entire plants and their organs. The association of cells, however, produces 

 very complex interactions whose complexity increases with the division of 

 labour and increasing specialization. In the absence of protoplasmic 

 connexions these influences, however complicated, are such as could be 

 maintained by external agencies of suitable character if appropriately applied, 

 whereas the existence of living continuity creates relationships of very special 

 nature. To the first-mentioned class belong more especially the chemical 

 and mechanical effects due to the plant's own activity, and also the influences 

 exercised upon the tissues by the performance of such functions as absorp- 

 tion, translocation, and transpiration. Electric currents of internal origin 



