472 SCIENCE PROGRESS. 



growing point, under normal conditions, is only adapted to 

 produce the particular order of branch to which it belongs ; 

 its development does not necessarily throw any light on the 

 way in which the structure first arose. Hence the theory 

 of recapitulation, however true of the development of the 

 plant as a whole, need not apply at all to the apical develop- 

 ment in a plant which is no longer an embryo. To the 

 neglect of this distinction we may perhaps attribute the fact 

 that the recapitulation-theory has never been adequately 

 tested in the vegetable kingdom. 



In considering the subject of histogenetic layers, we 

 have therefore to distinguish two questions : ( i ) Do such 

 layers exist in the embryo ? (2) Can they be traced in the 

 various growing points, at any stage of growth ? 



As regards the former question, it is probable, judging 

 from the comparatively few cases which have been investi- 

 gated, that in the main axis of the embryo such a differen- 

 tiation, into plerome, periblem and dermatogen, giving rise to 

 stele, cortex, and epidermis, does exist (see, besides the older 

 literature, Chauveaud, 17). There is no need to dwell on 

 these phenomena, which have been familiar to botanical 

 students for many years ; we can only wish that the con- 

 clusions rested on a somewhat broader basis. There is, 

 however, a fatal objection to anything like a theory of 

 "germinal layers" in plants, as was clearly pointed out by 

 De Bary in his criticism of Famintzin (1, p. 23). The 

 plerome of the embryo may give rise to the whole vascular 

 system of the main axis, but it certainly has nothing to do 

 with the origination of the same system in the lateral ap- 

 pendages of the stem. Such appendages are of exogenous 

 origin, and there is no doubt that their vascular tissues, like 

 all their other tissues, arise from the outer layers of meri- 

 stem of the parent organ, and not from its plerome. The 

 converse case occurs in the root, where the appendages are 

 endogenous. In Phanerogams all the tissues of a lateral 

 root arise from the pericycle, and thus belong to the 

 plerome of the main root, while in vascular Crypto- 

 gams they arise from the endodermis, and are thus all 

 alike of cortical origin. Hence it is impossible to derive 



