THE STELA R THEORY. 219 



monocotyledons and some dicotyledons, while Koch takes 

 the view that there is a generalised meristem without 

 separate layers in Gymnosperms (21) and that only the 

 dermatogen is separate in most Angiosperms (22). So 

 that the "triple layer" theory of Hanstein and Van 

 Tieghem is accepted by neither of these two most recent 

 investigators as of general application, widely divergent as 

 are their views inter se. Considering that the theory of 

 the direction of ontogeny by the separation of different 

 kinds of somatic idioplasm is now generally discredited, it 

 is difficult to see what we gain by an adherence to the un- 

 proved hypothesis of the strict separation of the initial 

 layers, even if it is still a possible hypothesis. 



In the root apex on the contrary the plerome is in the 

 great majority of cases sharply separated from the peri- 

 blem, but even this rule is not universal. The sharp 

 separation seems to be correlated both in root and stem 

 with the formation of a slender compact cylinder. 



In Vascular Cryptogams, which nearly all possess either 

 a single apical cell or a single layer of initial cells giving 

 rise to the whole of the tissue of the axis, there is of course 

 no question of a separation, at the apex itself, of initial 

 layers. 



The separation of the young cylinder behind the actual 

 growing point is quite a distinct question from its separation 

 at the apex. It is during the development of the cylinder 

 that we get, usually at least, a distinct limit between it and 

 the cortex which is often lost in the adult stem, and this is 

 a point of great importance. 



Long before the stelar theory was originated, most of 

 the great anatomists, who laid the foundations of our know- 

 ledge of the histology of vascular plants, were practically 

 agreed on the generality of this early separation. This is 

 clearly shown in the terminology employed in designating 

 the various regions. 



Thus Sanio (24), tracing from the apex the development 

 of the various tissues, showed that in many cases the young 

 pith first became separated from an outer zone, and that 

 in the latter the "thickening ring" (really corresponding to 



