STRATIFIED APICAL MERISTEMS 87 



has already been made. Each of these three sharply differentiated 

 histogens is derived either from a single apical cell or from several 

 apical initials, the latter arrangement being the more usual one. 



As a matter of fact, such a differentiation of the primordial 

 meristem is quite evident in the case of certain Phanerogamic stem- 

 apices. The most frequently quoted instance is that of Hippuris 

 vulgaris (Fig. 17). Here the dermatogen is clearly demarcated from 

 the five-layered periblem, and the latter is also obviously marked 

 off from the plerome, which can often be traced back to a single 

 initial cell. Other cases undoubtedly exist, to which Hanstein's 



Fig. 17. 



Longitudinal section through the growing-point of Ilippuris vulgaris. d, derma- 

 togen ; per, periblem ; pi, plerome ; i, plerome-initial ; b-lfi, primordia of successive 

 leaves. After Warming (from Schenck's Handbook-). 



classification is equally applicable : but in relation to the general 

 theory of apical growth by means of several initials outlined above, 

 Hanstein's classification amounts to nothing more than a summary 

 of the features common to certain special cases of such apical 

 growth, all of which can be quite naturally included in the second 

 or third of the principal types of arrangement. As a matter 

 of fact, the structure of the primordial meristem must conform to 

 Hanstein's scheme, wherever the following two conditions are fulfilled : 

 in the first place, there must be at least three vertically superimposed 

 initials or groups of initials present, and. secondly, the uppermost 

 initial or group of initials must not give rise to more than a single 

 layer of cells. The above-described apex of the young axillary shoot 

 of Ceratophyllum demersum with its three superimposed initials may 

 accordingly be regarded as the simplest instance of a growing-point 

 differentiated into plerome, periblem and dermatogen. 



