APICAL CELLS 



85 



segments, which on the one hand go to make up the root-cap, while on 

 the other they become incorporated in the body of the root. In 

 addition lateral segments are cut off by appropriately disposed longi- 

 tudinal walls. The products of the four apical cells arrange themselves 

 in quadrants, marked off from one another by walls which are thicker 

 and more continuous than the rest. - ' 19 



Schwendener states that four initials meeting at the centre of the 

 apical dome also occur in the stems of Conifers (vegetative shoots of 

 Juniperus communis, seedling axis of Pinus mops, P. Laricio, P. sylves- 

 tris, Abies alba). The occurrence of a single three-sided apical cell, 

 previously described by Dingier as 

 the normal condition among Conifers, 

 appears to be quite an exceptional 

 arrangement in this group of plants. 



The simplest form involving only 

 two initials of the second or vertical 

 type of seriation is exemplified by the 

 young leaf-segments of Ceratophyllum 

 (lemersnm (Fig. 15). According to the 

 author's observations the upper of the 

 two initials in this case takes the form 

 of a three- or four-sided pyramid, with 

 a convex outer face and a truncated 

 inner end ; if it is four-sided, the 

 walls are usually so arranged that 



two adaxial and two abaxial rows of segments result (Fig. 1 5 b). 

 These segments divide solely by anticlinal walls and undergo no 

 periclinal divisions whatsoever. Hence the shell of tissue derived 

 from them comprises a single layer of cells, and in fact consti- 

 tutes the outermost meristematic layer, which corresponds to the 

 primary dermal layer of the adult organ, and which on that account 

 has been termed the dermatogen by Hanstein. The second or lower of 

 the two initials, which is shaped like a complete three- or four-sided 

 pyramid, divides by oblique walls, in precisely the same manner as the 

 solitary initial in the stem of a Fern or Horsetail (Fig. 15 A, c). Each 

 of its segments becomes subdivided, by an approximately radial wall, 

 into two slightly unequal daughter-cells : further division of the latter 

 leads to the formation of a central mass of meristematic tissue- 

 enveloped in the dermatogen which gives rise to the whole of the 

 permanent tissue of the leaf, with the single exception of the 

 epidermis. 



The author has further shown that the apical growth of young 

 axillary branches of Ceratophyllum demersum is more complex, owing 



Fig. 15. 



Apex of a young leaf-segment of Cera- 

 tophyllum demersum. A. Longitudinal sec- 

 tion. B. Surface view. C. Optical trans, 

 verse section through the lower of the two 

 apical initials. 



