8 



INTRODUCTION. 



Fig. r. — (225) Hippuris vulgaris. Median longfitudinal 

 section through the apex of quite a youns; leafy shoot, 

 which had just begun to develope the second whorl of 

 leaves. In older shoots the apex is much more elongated. 

 C£ Sachs, Textbook, Engl. ed. 1882, p. 154. For further 

 explanation see Text. 



This diflferentiation of the meristem is retained at the punctiim vegetationis of the 

 first stem and of the tap root ; further it appears at the piinciwn vegetationis of all 

 lateral stems and secondary roots. The differentiation certainly varies greatly in 

 distinctness in different cases, and is complicated in roots by the presence of the 

 Calyptrogen-layer. It corresponds exactly to the scheme given above in the apex of 



the stem of many thin-stemmed water plants, 

 as Elodea, Hippuris, &c. ; in these Sanio ^ 

 first discovered the appearances in question, 

 which were afterwards more fully worked out 

 by Hanstein ". The rounded conical apex of 

 the puncium vegetationis of Hippuris (Fig. i) 

 is covered by a single layer of dermatogen (e). 

 Then, passing inwards, follow usually five 

 regularly concentric layers of isodiametric 

 cells, these constitute the periblem which 

 surrounds the plerome-cylinder {p-p). The 

 latter has a bluntly conical apex, where it is 

 often terminated by a single cell, while below 

 it widens out. In most other cases the num- 

 ber of periblem-layers is smaller, only 2-3. 

 The three zones of meristem, though they remain distinct, equally take part in 

 the acropetal growth in length of the end of the stem. Each is continually renewed 

 by divisions in the group of cells (or single cell) which forms their apical portion, 

 while as the tissues are removed from the apex the transition from meristem 

 to the definitive tissue takes place. Each is continuous downwards into definite 

 tissues or tissue- systems, which will be mentioned hereafter. That apical group of 

 cells, or (as in the plerome of Hippuris) single cell, which renews the layer and 

 which hence always introduces the further cell-divisions in it, is called the Initial cell 

 or Initial group of the layer. 



In all Angiospermous plants the dermatogen layer is marked off with similar 

 sharpness from the tissues below it, and is distinguished by its division walls being 

 arranged only at right angles to the surface ; with this restriction they run in all 

 directions. But the separation of plerome and periblem does not appear in all 

 cases so sharply marked as in the foregoing instance. Especially where the apex 

 of the stem is broad and flat, it must often be left undecided whether both do not 

 originate from a single common initial group, and are first clearly separated at some 

 distance from the apex, on their gradual transition to definite systems of tissue. 



The origination of the normal lateral branches of the end of the stem (i. e. of 

 the leaves and lateral shoots) as emergences of the surface begins beneath the 

 apex by the outgrowth of definite groups of meristem, which were not previously 

 to be distinguished by any special character. These are the initial groups of the 

 emergence. Asa matter of fact, both elements of the dermatogen-layer and of the 

 periblem lying under it take part in the origination, the growth and divisions of both 

 proceeding simultaneously (comp. Fig. 1) ; but the cells belonging to the dermatogen 



* Bot. Zeitg. 1S64, p. 223. 



[Kn)', Botan. Zeitg. 1878, p. 760.] 



