1 78 SEGMENTATION 



generalisation came to be widely accepted that the delimitation of the 

 embryonic tissues by Hanstein should coincide exactly with that of the 

 mature tissues by Van Tieghem, and that this is generally applicable to 

 stems as well as roots of Vascular Plants. 



But this whole subject has recently been submitted to a detailed 

 revision by Schoute, 1 and it has been shown that the generalisation of 

 Hanstein that the three formative regions exist at the apex was based on 

 a very narrow area of observation. As a matter of fact, there is no separate 

 origin of them in Pteridophytes, for they all spring from the initial cell or 

 cells. In the Phanerogams such separate origin is best seen in roots, but 

 even there it is not constant at the extreme tip. In stems it is only seen 

 clearly in some few cases ; in most stems hardly at all. The dermatogen 

 is the most frequently and the most clearly defined of the three : the 

 distinction of periblem and plerome in stems is only rarely carried out. 

 Neither is the partitioning of the formative regions in the embryo clearly 

 marked at an early stage in vascular plants at large : though there is some 

 approach to it in some Dicotyledons, the Monocotyledons and Gymno- 

 sperms show little regularity, and it is almost entirely wanting in some 

 plants. Thus it cannot be said that the details of development of the 

 embryo in any way strengthen the position as regards the formative regions 

 of Hanstein. The general conclusion seems a justifiable one, that no 

 great morphological importance is to be attached to the formative regions 

 of Hanstein since they are so commonly of inconstant occurrence. 



An examination of the further question whether the dermatogen really 

 produces epidermis, the periblem cortex, and the plerome the central 

 cylinder, also shows inconstant results. In those roots in which periblem 

 and plerome are clearly defined at the apex the cortex does originate 

 from the periblem and the central cylinder from the plerome, though this 

 does not hold exactly for all roots. But in stems the correspondence of 

 Van Tieghem's primary tissue-systems with the formative regions does not 

 hold : it is not even carried out exactly in the single regular example 

 which has been described, viz. Hippuris : for here, according to Schoute, 

 the endodermis and part of the cortex may be derived from the plerome. 

 It is thus seen that the case is similar to that already discussed of the 

 relation of apical segmentation to the origin of the appendages : in certain 

 few examples the early segmentation may coincide with the definite mature 

 condition, but in the great majority no such regular relation exists in 

 either case. Where it does exist it may be held to be casual rather than 

 causal, and will bear no constant phylogenetic significance. 



The reasonable degree of success which seemed at first to attend these 

 efforts to correlate with early segmentation at the apex not only mature 

 external form, but also internal structure, led to a peculiar development 

 in the study of the primary origin of the embryo from the egg. However 

 clearly we may now see that the position assumed by the investigators of 



1 Die Steldr Theorie^ Groningen, 1902. 



