476 VENTILATING SYSTEM 



palvstris, Menyanthes trifoliata). Complete and permanent protection 

 is ensured, when the guard-cells never separate entirely from one 

 another, so that the stoma remains imperforate. In some of these 

 cases the external cuticular ridges remain fused {Potamogeton nutans) ; 

 in others the guard-cells are completely joined together, with the sole 

 exception of the internal cuticular ridges {Polygonum amphihivm, 

 Schoenoplectiis lacustris). In either case, the stoma presents a very 

 characteristic appearance in transverse section. 



The pores of the stomata are the only perforations in the typical 

 epidermis ; hence it is hardly surprising that these structures are some- 

 times utilised as outlets for liquid secretions. Illustrations are provided 

 by water-stomata, and by the so-called " sap- valves " of nectaries. The 

 alleged secretion of wax by stomata, in the case of certain fruits (e.g. 

 Cyclonia japonica, Rosa glandulosa, Primus ccrasus, etc.), requires further 

 investigation. 



A very remarkable instance of change of function discovered by 

 the author and examined more recently by Bobisut is furnished by 

 the modified stomata which are found on the slippery region of the 

 pitcher of Nepenthes. Goebel was the first to draw attention to the 

 presence, on this slippery wax-covered tract of the pitcher, of small 

 cells of crescentic outline with their convex sides facing upwards. The 

 author himself next pointed out, that the shape and arrangement of 

 these cells is such, that they assist insects to crawl down the inside 

 of the pitcher, but offer no foothold to those that attempt to retrace 

 their steps. It has now been shown [by Bobisut], that these peculiar 

 cells are nothing more nor less than the raised guard-cells of trans- 

 versely elongated stomata, which have [quite given up their original 

 function] and have become transformed into structures accessory to 

 the capture of insects. Nepenthes Rajflesiana illustrates various stages 

 in this remarkable process of modification (Fig. 189), which depends 

 essentially upon the fact that the upper member of each pair of guard- 

 cells [i.e. that nearest the mouth of the pitcher] projects a good deal 

 further than its fellow, so that the whole stoma acquires a very asym- 

 metrical shape. As seen in a vertical section, the upper guard-cell or 

 " clamp-cell " does not appear very greatly modified ; its outer cuticular 

 ridge is normal, or even slightly over-developed ; the internal ridge, 

 however, is entirely absent (Fig. 189 a, b, c). The lower guard-cell, 

 which remains Hush with the general level of the epidermis, undergoes 

 a far greater alteration of shape. Here the outer cuticular ridge is 

 rarely normal, being generally more or less obviously reduced or alto- 

 gether suppressed ; in the latter case, the " guard-cell " is barely dis- 

 tinguishable from an ordinary epidermal element (Fig. 189 c). The 

 front-cavities of these modified stomata thus persist, though their shape 



