332 SCIENCE PROGRESS 



three strata, during the growttfof the plant, are in an active state 

 of division, and give origin to the tissues found in the mature 

 parts of the plant. Hanstein supposed that of these three layers, 

 or histogens, the dermatogen gave rise to the epidermis, the 

 periblem to the cortex, and the plerome to the central cylinder. 

 This is often spoken of as the theory of histogens. 



Now if the cortex be formed from the periblem and the central 

 cylinder from the plerome, then the matter can be cleared up by 

 tracing the fate of these histogenic layers. 



This important task was undertaken and acccomplished by 

 Schoute (49), who discovered that it was the exception rather 

 than the rule for the endodermis to be absent. He found that 

 only seven plants out of four hundred Dicotyledons had no 

 endodermis. Thus, to a certain extent, the stelar theory may be 

 supported ; but it must be pointed out that this tissue may only 

 occur in young stages of development. As regards the coin- 

 cidence of the pericycle with the outermost layer of the plerome, 

 and of the endodermis with the innermost layer of the periblem, 

 Schoute has ascertained that this does not hold in the majority 

 of the cases examined. In some plants the plerome not only 

 gave rise to the pericycle, but also to the endodermis together 

 with some of the inner cortical elements ; in other cases, the 

 plerome, as such, was non-existent. 



Thus, incidentally, Hanstein's theory of histogens is de- 

 stroyed, and, to the minds of some, the endodermis, considered as 

 a layer of morphological value, shares a similar fate. 



Therefore it is clear that much of Van Tieghem's theory 

 is discredited ; and although the majority of botanists have 

 discarded his hypothesis, taken as a whole, two important 

 conclusions have been retained in many quarters, viz. : 



(1) That the stele, not the vascular bundle, is the unit of 

 structure. 



(2) That a simple type of monostele (the protostele) is the 

 primitive form. 



From these two ideas new views have arisen regarding the 

 stele. 



Jeffrey (37, 38) considers that there are two types of central 

 cylinder, the protostele and the siphonostele, the former being 

 the more primitive. The siphonostele is tubular, has a medulla 

 derived from the cortical tissue, and has given successive origin, 

 by reduction, to the following modifications : the amphiphloic 



