THE INKER TISSUES OF PLANTS. 283 



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

 tion 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 would be if so produced; and that their terminations 

 in the tissue of the parent shoot are partially-formed lines 

 of irregular elongated cells, like those out of which the ves- 

 sels 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, need not be dis- 

 cussed : 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 dehiscences 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 

 repetition 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 surfaces, may consist either of successive rings, or con- 



