INTRODUCTION TO PLANT HISTOLOGY 



451 



and soma cells which Weismann suggested in animals. Theoretically every 

 cell of the plant is totipotential, that is, it retains in itself the possibility of 

 regenerating the whole plant, in suitable circumstances. Any actual limita- 

 tions of this power are imposed by special conditions. The differentiation 

 of cells is therefore explicable, not by inherent differences between them 

 but by the influence of their environment, that is to say by differences of 



Dermatogen 



Periblem 



Plerome 



Apical meristem 

 Calyptrogen 



Calyptra 





».^,. 





Fig. 439. — Vicia faba. Longitudinal 

 section of root apex showing the 

 histogens. 



water and food supply, oxidation, etc., due to the position of the cell in the 



tissues. 



Even within the meristem some difi"erentiation is apparent. In 1868 

 Hanstein named three zones, which he considered to be typical and which 

 he called histogens or " tissue producers " (Fig. 439). The outermost was 

 a single layer of cells named the dermatogen, which gave rise to the 

 epidermis. Beneath this came an intermediate zone, the periblem, giving 

 rise to the cortex and the inner tissues of the leaf. Centrally was the core or 

 plerome, producing pith and vascular tissues. In cases where such layers 

 can be seen these names still have a descriptive value and are frequently 

 used, but the developmental significance assigned to them by their author 



