THE STELA R THEORY. 145 



various strands is quite a distinct matter, and requires a 

 distinct terminology. 



THE BOUNDARY OF THE STELE. 



The acceptance of the central cylinder in the " mono- 

 stelic " stem as a region of the first morphological rank is 

 now very general. The only criticism which we have to 

 consider is that which calls attention to the frequent want 

 of definiteness about its external limit, and is inclined on 

 this ground to question its individuality. This want of 

 definiteness arises from the absence, in many adult stems, 

 of the special characters of the endodermis (innermost 

 layer of the cortex), often combined with an identity in 

 size, shape and characters of cell-membrane between 

 the cells of the cortex and those of the conjunctive. Such 

 a state of things obtains, to take a single instance, in the 

 stem of Ranunculus repens. A transverse section of such 

 a stem shows the separate bundles imbedded in a homo- 

 geneous ground tissue, and to speak of a well-marked central 

 cylinder is to speak of that which does not, in fact, exist. 



Now this, as it stands, is a perfectly legitimate criticism, 



and its force as against the general validity of the stelar 



idea depends simply upon the greater or less generality of 



the condition described. Van Tieghem (10, p. 752) states 



that when, after the formation of the endodermis, the stem 



undergoes considerable intercalary growth, the folds on 



the radial walls of the endodermal cells become stretched out 



so that they become difficult or impossible to see. In other 



cases no suberisation of the radial walls occurs, and then, 



unless the endodermal cells are distinguished by possessing 



starch, it is admitted that the limit of the cylinder is difficult 



to determine, but says Van Tieghem {Joe. eit.) : " il reste la 



forme differente des cellules ". This, however, as has been 



said, is by no means always obvious. A possible cause of 



such a condition, assuming the limits of the young cylinder 



to be well defined, has already been suggested, but 



new investigations are necessary to determine the point. 



If, for the sake of argument, we make the opposite assump- 



10 



