CAMBIUM AND MERISTEM. 15 
In considering the different forms of stems, it is of the greatest 
otyledon we have cambiu 
of meristem, all containing cells capable of multiplying: by division. 
There is the phellogen, or cortical meristem, the meristem forming the 
ndles. Th 
medullary rays cokenye analy Naegeli), or, in other words, toca 
rming a uniform zone, while the bark increases in thickness by the 
ffination of new cells (chiefly cork) by the periblem meristem 
In monocotyledons the procambium does not form cambium, the 
whole of the tissue forming the permanent cells and vessels of the 
bundle. In some stems the plcrom ee layer is well developed, 
as, for example, in Dracena.* In monocotyledons the periblem tissues 
are but slightly developed. Rear: the periphery of the stem se 
n of 
rm, 
- undles develope, in Dracena new cane 
strings form, and thus both plerom “pereaghymne (pith) and new fibro- 
vascular — s, with their varied forms of tissue, are produced 
In vascular cryptogams no dsarintogen forms, the two elements, 
plerom ont periblem, alone existing. The plerom tissues seem early 
to pass into permanent tissue, no cambium or meristem remaining 
The periblem tissues are, however, vonmore'd developed. The avian 
layer differentiates into an epidermis with its appendages, while the 
periblem meristem may be largely Senlios ed, as in Jsoetes, in which 
circumferential growth is seen to take Ha th 
In the gi 
1Z say that this gro is 
exogenous, meaning that the growth rates that of a dicotyledon 
or scepobisnerms seems a mistake, because it is on the periblem 
istem, and not on the cambium and plerom meristem, that the 
pone depends. In most archisperms, as in the vascular cryptogams, 
no dermatogen is formed, the primitive meristem differentiating into 
periblem and plerom " 
Sachs divides the tissues of plants into three groups—epidermal 
tissues, fibro-vascular bundles, and primitive tissue (grundgewebe). 
The latter form must be abandoned, because it belongs both to 
the periblem and gi and I be lieve the most satisfactory divi- 
s’ Lehrbuch, ed. 2, y “te fig. 90. 
7 Lebsbach. ed. 2, p. 74, e 
