96 



MERLSTEMATIC SYSTEM 



undergoing appropriate divisions, also produce photosynthetic cells or 

 other parenchymatous elements. The disposition of procambial groups, 

 as seen in a transverse section through a young organ, naturally corre- 

 sponds to the primary arrangement of bundles in the adult organ. The 

 procambium is thus often disposed in isolated longitudinal strands, for 

 instance, in the stems of a great many Monocotyledons. Most roots, 



Fig. 2-2. 



The primary meristems. A. Part of the adaxial half of a transverse section through 

 a very young leaf of Pandanus utilis. B. Part of a radial longitudinal section 

 through the same leaf, X400. C. Part of a transverse section through a young 

 petiole of Asplenium foecundum, x 125 ; p, protoderm ; c, procambium; m, funda- 

 mental meristem. In C the cells of the protoderin are undergoing tangential 

 division, while the fundamental meristem is beginning to be converted into 

 chlorophyll-containing parenchyma. 



on the other hand, are provided with a single central procambial strand. 

 In other cases again, the procambium forms a continuous hollow cylinder, 

 which is not infrequently supplemented by variously-arranged isolated 

 strands. 



The procambium always arises from longitudinal series of 

 primordial mother-cells, in which further division takes place only by 

 longitudinal walls, situated in various planes. To begin with, therefore, 

 it consists of small prismatic cells with ordinary transverse walls. 

 Very soon, however, the individual cells of the procambium undergo 

 active apical growth, as a result of which their terminal walls 

 become very oblique ; the cells thus acquire the gable- or awl- 



