268 PHYSIOLOGICAL DEVELOPMENT. 



growing parts in anticipation of functions to be afterwards 

 discharged. But, without alleging any. special cause for this 

 metamorphosis, there is good evidence that it is in some way 

 consequent upon the carrying of sap. If we examine tissues 

 such as that in the interior of a growing turnip that has 

 not yet become stringy, we may, in the first place, find 

 bundles of elongated cells not having yet developed in them 

 those fenestrated or reticulated structures by which the ducts 

 are eventually characterized. Along the centres of adjacent 

 bundles we may find incomplete lines of such cells — some that 

 are partially or wholly transformed, with some between them 

 that are not transformed. In other bundles, completed chains 

 of such transformed cells are visible. And then, in still 

 older bundles, there are several complete chains running side 

 by side. All which facts imply a metamorphosis of the 

 elongated cells, caused by the continued action of the currents 

 carried. 



§ 281. Here, however, presents itself a further problem. 

 Taking it as manifest that there is a typical distribution of 

 supporting tissue adapted to meet the mechanical strains a 

 plant is exposed to by its typical mode of growth, and also 

 that there goes on special adaptation of the supporting tissue 

 to the special strains the individual plant has to bear ; and 

 taking it as tolerably evident that the sap channels are 

 originally determined by the passage of currents along lines 

 of least resistance ; there still remains the ultimate question — 

 Through what physical actions are established these general 

 and special adjustments of supporting tissue to the strains 

 borne, and these distributions of nutritive liquid required to 

 make possible such adjustments ? Clearly, if the external 

 actions produce internal reactions ; and if this play of actions 

 and reactions results in a balancing of the ^strains by the 

 resistances ; we may rationally suspect that the incident 

 forces are directly conducive to the structural changes by 

 which they are met. Let us consider how they must work. 



