266 PHYSIOLOGICAL DEVELOPMENT. 



sections be made through a growing bud of Opuntia or 

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

 mation converge towards the point of growth, as they would 

 do if the sap-currents determined their formation ; that 

 they 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 partially- 

 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 destroyed by the passing current, is one for 

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

 by the liquids they transmit, or whether they 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, disappear, 

 leaving internal ridges showing their positions ; and, in other 

 cases, become extremely porous. Though it is manifest that 

 this is not the process of vascular development in tissues that 

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

 eccnces or perforations of septa occur before such direct 

 actions can have come into play ; yet it is still possible 

 that the disappearances of septa which now arise by repe 

 tition of the type were established in the type by such 

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



simultaneous change undergone by these longitudinally - 

 united cells must be otherwise caused. Frame-works are 

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



