ONTOGENY 



483 



IV ONTOGENY OF THE VENT1IATINO SYSTEM. 



Though the ventilating system is always composed of air-containing 

 intercellular spaces, it may develop in a great variety of ways, just 

 like other tissue-systems. As a matter of fact, the majority of venti- 

 lating spaces arise by a process of separation which depends upon the 

 splitting of a common primary cell-wall or middle lamella. The air- 

 passages and -cavities of the Nymphaeaceae, and of Papyrus antiquorum, 

 lemna, Trapa, Potamogeton, Geratophylhtm, etc., have this schizogenous 

 origin. In other cases ventilating spaces 

 originate lysigenously (rhexigenously), i.e. by 

 the collapse and disintegration of transitory 

 tissues, which previously become pervaded by 

 narrow schizogenous spaces; this case is illus- 

 trated by the air-passages of most Cyperaceae, 

 Gramineae, and Equisetales. As regards the 

 details of development, both lysigenous and 

 schizogenous ventilating spaces display the 

 greatest diversity. 237 



Aerenchyma sometimes originates from 

 fundamental meristem, but in other cases arises 

 from a secondary meristematic layer, which is 

 the phylogenetic equivalent of the phellogen. 



The cells which compose the stomatic 

 apparatus, in the wide sense, pertain either 



to the protoderm or to the fundamental meristem. The guard-cells 

 are, of course, always sister-cells, and invariably arise from a 

 protodermal mother- cell. As a rule, however, certain preparatory 

 divisions, which give rise to the subsidiary cells of the stomatic 

 apparatus, precede the segregation of the actual guard-cells. 23 The 

 details of these preliminary divisions vary considerably in different 

 cases ; it must suffice here to refer briefly to the development of 

 stomata in two specific cases, namely in the Crassulaceae and in 

 Mercurialis (Fig. 192). In both these instances the protodermal 

 primordial mother-cell of the stoma divides after the fashion of an 

 apical cell; in the Crassulaceae the segments are cut off in three 

 planes, while Mercurialis adheres to the scheme of a two-sided apical 

 initial. 



Lenticels may arise either upon young stems, which still retain 

 their epidermis, or upon older branches, after the formation of peri- 

 derm has begun. Their development, in the former case, is described 

 by Stahl as follows. The parenchymatous cells adjoining the internal 

 air-chamber of a stoma increase in size and divide ; the resulting 



Fig. 192. 



Stoma and subsidiary cells of 

 Sempervivum, sp. The numerals 

 indicate the course of segmen- 

 tation. 



