84 MERISTEMATIC SYSTEM 



In the first mode of arrangement the apical cells are all located in 

 the same horizontal plane: they are, in fact, juxtaposed or horizontally 

 seriated. In this case they must all abnt directly against the median 

 plane in a longitudinal section; as seen in horizontal projection they 

 must be grouped around the axis, or, in other words, around the centre 

 of the apical dome. Here the primordial meristematic tissues take the 

 form of longitudinal strips derived from the several initials. 



In the second case the apical cells are superimposed or vertically 

 seriated, that is to say, they are situated in successive horizontal planes. 

 In a radial section they are all seen to lie in the median plane, while 

 in relation to the whole apical dome they are, of course, central. In 

 this case the lowermost initial gives rise to an axile strand of primor- 

 dial meristem, which represents, as it were, the core of the growing- 

 point : the others, on the contrary, produce mantle-like meristematic 

 layers, which form so many concentric shells around the axile strand. 

 Each of these shells grows by means of an independent initial. 



In the third and most complex type of arrangement the apical 

 cells are both juxtaposed and superimposed. In other words, each of 

 the successive horizontal planes includes a group of initials ; every 

 group must of course comply with the conditions laid down in the case 

 of the first type for the single layer of initials. Since this plan 

 obviously represents a combination of the first and second types of 

 structure, it is hardly necessary to consider in detail the arrangement 

 of the meristematic layers that arise from the several initials. It may, 

 however, be noted that a stratification of the meristem due to the 

 vertical seriation of the initials is sometimes evident for some distance 

 behind the apex, whereas the horizontal seriation produces no such 

 effect. 



Each of the above three principal types of seriation includes a num- 

 ber of sub-types, which differ from one another in points of detail. Some 

 of these varieties of arrangement may next be illustrated by examples. 

 The simplest illustration of horizontal seriation of initials is furnished 

 by the stem-apex of Selaginella Wallichii, which has been described by 

 Strasburger. Here there are two wedge-shaped apical cells, of which 

 one produces the right and the other the left half of the dorsiventral 

 stem. The apical faces of these cells are narrow and rectangular, 

 while their lateral surfaces are broadly triangular and extended at 

 right angles to the plane of dorsiventrality. The segments are cut oh 

 in four vertical rows. 



According to Schwendener four initial cells are grouped around 

 lln" centre of the apical dome in the roots of MARATTIACEAE. A 

 radial longitudinal section reveals two initials, one on either side of 

 the middle line : each of these gives rise by periclinal divisions to 



