86 



MERISTEMATIC SYSTEM 



to the presence of three superimposed initials (Fig. 10). The upper- 

 most of these gives rise, just as in the leaf, to a single superficial 

 layer of meristem. The second or middle cell at first behaves in a 

 similar fashion ; but later, as the apical dome enlarges, periclinal 

 as well as anticlinal walls appear in this second shell, so that the 

 final product of the middle initial is a many-layered meristem. The 

 third or lowermost initial, finally, divides by oblique walls and pro- 

 duces an axile mass or core of meristematic tissue. 



In the case of the third type of arrangement of apical cells [where 

 the initials are both juxtaposed and superimposed] the difficulties 



of observation are so great, that no 

 instance has as yet been thoroughly 

 worked out; it is highly probable, how- 

 ever, that this complex form of apical 

 growth is widely distributed among 

 Angiosperms. 40 



It may, in any case, be regarded as 

 certain that, among Phanerogams, the 

 mode of apical growth is liable to 

 considerable variation not only within 

 a species but even in one and the same 

 shoot. Thus the primary axis of the 

 seedling often differs in this respect 

 from a subsequently formed lateral 

 shoot, while the latter in its turn may 

 exemplify one type of arrangement 

 of the apical initials when young, 

 and a different type later on. The 

 fact that different investigators so 

 frequently disagree concerning the apical growth of Phanerogams is no 

 doubt to a large extent attributable to this variability. As has already 

 been pointed out, the apical cells of certain Pteridophyta display a 

 similar inconstancy of behaviour. 



The discussion of apical growth by means of several initials 

 naturally leads to a consideration of Hanstein's classification of the 

 apical tissues of Angiosperms. 41 Hanstein assumes that the primordial 

 meristem is always sharply separable into three distinct strata or 

 " histogens." He distinguishes a central strand of plerome, composed 

 mainly of cells extended in the longitudinal direction : this plerome is 

 surrounded by a shell of periblem which usually consists of several 

 strictly concentric layers of isodiametric cells. The periblem in 

 turn is succeeded by a second shell of meristematic tissue, comprising 

 a single layer of cells : this is the dermatogen, to which reference 



Fig. 16. 



Median longitudinal section through a 

 young axillary branch of Ceratophyllum 

 '/ un rsum, 



