266 PHYSIOLOGICAL DEVELOPMENT. 



sections be made through a growing bud of Opuntia or 

 Cereus, it will be found that the vessels in course of for- 

 mation converge towaids the point of growth, as they would 

 do if the sap-currents determined their formation ; that 

 the}'' are most developed near their place of convergence, 

 which they also would be if so produced ; and that their 

 terminations in the tissue of the parent shoot are partial! 3^- 

 formed lines of irregular fibrous cells, like those out of 

 which the vessels of a leaf or bud are developed. 



Concluding, then, that sap-vessels arise along the lines of 

 least resistance, through which currents are drawn or forced, 

 the question to be asked is — What physical process produces 

 them ? Their component cells, united end to end more or less 

 irregularly in ways determined by their original positions, 

 form a channel much more permeable, both longitudinally 

 and. laterally, than the tissue around. How is this greater 

 permeability caused ? The idea, first propounded 



I believe by "Wolff, that the adjoined ends of the cells are 

 perforated or destroj'ed b}^ the passing current, is one for 

 which much is to be said. Whether these septa are dissolved 

 by the liquids they transmit, or whether the}^ are burst by those 

 sudden gushes which, as we shall hereafter see, must frequently 

 take place along these canals, needs not be discussed : it is 

 sufficient for us that the septa do, in many cases, disajopear, 

 leavino^ internal ridges showing their positions ; and, in other 

 cases, become extremely porous. Though it is manifest that 

 this is not the process of vascular devel.ipment in tissues that 

 mifold after pre-determined types, since, in these, the dehi- 

 scences or perforations of septa occur before such direct 

 actions can have come into pla}" ; yet it is still possible 

 that the disappearances of septa which now arise by repe- 

 tition of the type were established in tlie type by such 

 direct actions. Be this as it may, however, a 



simultaneous change undergone by these longitudinally- 

 united cells must be otherwise caused. Prame- works are 

 formed in them— frame- works which, closely fitting their inner 



