476 SCIENCE PROGRESS. 



which at one time gives rise to cortex must at another 

 phase of growth contribute to the plerome. Koch's ob- 

 servations seem fatal to the theory of histogenetic layers 

 as regards the plants in question. 



In Angiosperms, as well as in Gymnosperms Koch 

 believes that Hanstein's plerome is nothing- but the young 

 pith (15, pp. 394, 401). He does not, however, find a 

 distinct initial group, even for the pith, though that is the 

 first tissue to become recognisable. According to his 

 views, the embryonic tissue of the apex grows as a whole, 

 and neither the pith nor any other tissue has any need of 

 a special histogenetic layer. 



The conclusion which appears to follow from our pre- 

 sent knowledge of the development of growing points, is 

 that the lines of cell-walls nearest the apex do not necessarily 

 mark the limits of the most important tissue-systems. As 

 Sachs long ago showed, the direction of the cell-walls is 

 quite independent of the nature of the organ ; a young hair 

 and a young embryo may present an exactly similar arrange- 

 ment of their cells (22). 



In Caulerpa differentiation goes on without the forma- 

 tion of any cell-walls at all ; in such Algae as Stypocaulon, 

 though segmentation occurs, morphological differentiation 

 (branching) begins in the apical cell itself, before any cell- 

 walls have been formed, while in the closely allied Sphace- 

 laria the reverse is the case. So too with the internal mor- 

 phology ; the systems of tissues may begin to differentiate 

 at a greater or less distance from the actual apex ; their 

 boundaries may or may not follow those lines of cell-walls 

 which happen to be first formed. 



An interesting paper by Flot (23) has recently appeared, 

 bearing on the differentiation of the tissues. The paper is 

 written from the point of view of Hansteinand Douliot, but 

 this does not essentially affect his main results. Flot's chief 

 conclusion (based principally, though not exclusively, on the 

 investigation of Dicotyledons) is that there is a definite 

 " perimedullary zone" within the ring of vascular bundles, 

 corresponding in many respects to the pericycle at their 

 outer margin. This perimedullary zone is quite distinct 



