SECONDARY MERISTEMS 99 



of thin-walled parenchyma which are best fitted to give rise to 

 secondary nieristems. Histologically considered, namely, parenchyma 

 has made less progress than any other type of permanent tissue 

 beyond the condition of the primary meristem from which it is 

 derived. As a matter of fact, the majority of secondary nieristems 

 do arise within parenchymatous mother- tissues. 



Ordinarily, secondary nieristems are developed either for the 

 purpose of effecting a secondary growth in thickness of the stem or 

 root, or in connection with the production of cork. The normal 

 secondary nieristems therefore are the interfascicular cambium and 

 the phellogen ; both these tissues are discussed more fully in sub- 

 sequent chapters. 



In the case of wholly adventitious organs, even the " primordial " 

 meristem may be secondary as regards its origin. Thus Hansen 

 has shown, that the adventitious shoots which develop upon severed 

 leaves of Begonia arise from adult, functional epidermal cells, through 

 the mediation of a secondary meristem. Those adventitious buds on 

 the other hand which appear on the upper or lower sides of the 

 fronds in certain Ferns (Asiokniinn bulbiferum, A. viviparity, Cerato- 

 pteris thalictroicles, etc.) originate from the protodermal layer, while the 

 leaf is still quite young. In this case, according to Heinricher, a single 

 element of the protoderm gives rise, by appropriate divisions, to a 

 three-sided initial, which becomes the apical cell of the adventitious 

 shoot. 45 



In conclusion, attention may be drawn to the fact which is 

 not without significance that the difference between primary and 

 secondary nieristems is by no means so well-marked as might be 

 inferred from the above-accepted definition of a typical secondary 

 meristem. As a matter of fact, these two conditions of the meristem 

 are connected by a complete series of intermediate stages ; in other 

 words, a primary meristem can become secondarily meristematic at 

 any stage of its transformation into permanent tissue. This point 

 is illustrated in the most instructive manner by the varying relations 

 which exist between the (primary) procambium and the (secondary) 

 cambial layer. 46 



