22 INTRODUCTION. 



Further ihey divide by longitudinal walls, which are similarly directed to their lateral 

 faces but are otherwise apparently irregularly arranged, into daughter cells, of which 

 those nearest the apical point always retain the properties of the common initial cells, 

 the others, as they retreat from the apical point, form the peripheral layers of meri- 

 stem, dividing first by numerous repeated tangential longitudinal walls, which are fol- 

 lowed by others in radial and transverse directions. In this case also a separation 

 between periblem and dermatogen appears first in an advanced stage of development. 



Lastly, we may notice shorlly the meristematic apex of Psiloium, which, according 

 to Strasburger, shows according to the quality of the shoot, either a simple apical cell, 

 or an initial group consisting of many members. Reference may be made to the 

 investigations of Nageli and Leitgeb, and of Strasburger *. 



The foregoing summary shows, first, that the similar differentiation of the 

 nieristem at the apex of stems and roots originates in a different way, that is, from 

 dilTerent first beginnings, in the different groups of the vegetable kingdom, and in 

 such groups as the Selaginellas and their allies, which are intermediate between the 

 larger divisions ; it originates differently even in the single species. 



Returning to the question, whether in all cases only definite zones of meristem 

 give rise to definite sorts of tissue, the most general answer, according to our" present 

 knowledge, is a distinct negative ^. To be sure this negative does not hold for all 

 single cases. For instance, for the large majority of roots, not only does each of the 

 different layers of meristem correspond to a definite section of a definite system of 

 tissue, but even the separate parts of each of these sections may often be traced back 

 to its separate initial cells in the apical meristem. It is therefore in this case not only 

 allowable, but preferable, for the sake of clearness, to term the layers of meristem 

 directly the initial layers of the axile vascular strand and its parts, or of the Epidermis, 

 &c., instead of using the terms selected above. But even in Roots exceptions occur. 

 The Epidermis, for instance, in the Gymnosperms does not originate from a distinct 

 d'ermatogen layer, so that we should properly speak of a Pseudo-epidermis, if we 

 regard as true Epidermis only the layer of cells derived from a distinct dermatogen 

 layer. In the aerial roots of most Orchids there arises from a distinct dermatogen 

 layer, as will be hereafter shown, a sort of tissue different from Epidermis. 



The negative, however, of the constant genesis of definite sorts of tissue, or 

 systems of tissue, from definite zones of primary meristem, holds to a much greater 

 extent for leaf-forming shoots. Here also it is true there are such relations. The 

 system of vascular bundles of many stems of Phanerogams, for instance, is derived 

 exclusively from the plerome cylinder. The plerome cylinder of the Lycopodiaceae 

 is transformed into the axile vascular cylinder ; dermatogen means in the Phanerogams 

 nothing further than young Epidermis, &c. But exactly the opposite also occurs. 

 The plerome cylinder arising from the inner cells of the segments develops in Azolla 

 (and SalviniaX) into the vascular bundle of the stem. In the stem of Equisetum^ it 

 develops into the — chiefly transient — axile cylinder of Parenchyma, and the system 

 of vascular bundles develops, according to the data at harid, exclusively from the zone 

 of periblem. And the whole of the tissues, and tissue-systems of the leaves, which 



' Botan. Zeitg. 1873, p. 118. 



- [Cf. Haberlandt, Entwickelungsgeschichte des mechanischen Gewebesystems d. Pflanzen, 1879.] 



^ Compare Sanio, Botan. Zeilg. 1864, p. 224. 



