PNEUMATHODES 445 



that these roots are provided with a genuine breathing mechanism, 

 involving movements of inspiration and expiration. This suggestion 

 certainly seems very plausible, but has not, so far, been put to the 

 test of experiment. 



In conclusion, it must be noted, that air-containing intercellular 

 spaces do not necessarily always serve for purposes of ventilation. It 

 frequently happens, that an axial or foliar organ, such as a Grass-haulm, 

 a scape of Leontodon or a leaf of Allium, is traversed by a single wide air- 

 passage ; in this case the hollow type of construction is determined by 

 mechanical considerations, and has no direct connection with ventila- 

 tion. Mention has already been made of the fact that, in photosynthetic 

 tissues, the intercellular clefts are arranged so as to ensure that the 

 synthetic materials shall emigrate by the shortest path. Similarly, 

 attention has been drawn to the fact that air-chambers and other 

 ventilating spaces are responsible for the buoyancy of floating plants. 

 Among those strand-plants, again, which depend upon ocean currents 

 for the dispersal of their seeds and fruits, the testa or pericarp is often 

 provided with a specialised floating tissue containing numerous inter- 

 cellular spaces ; the sole function of these air-spaces is reduction of 

 the specific gravity of the seed or fruit (cf. Ch. XI.). In white petals, 

 finally, and similar attractive organs, the ventilating function of the 

 air-spaces is of minor importance, in comparison with their ecological 

 value ; for here the attractive white coloration is due to the total 

 reflection of light by the air contained in the intercellular spaces. 



III. THE EXTERNAL OUTLETS OF THE VENTILATING 



SYSTEM. 



Direct communication between the ventilating system and the 

 outer atmosphere is rendered possible by the fact, that the dermal 

 tissues are interrupted at certain points by open ventilating pores or 

 pneumathodes,' 219 through which gases can pass freely in both directions. 

 There are three principal types of pneumathode, namely, the epidermal 

 stomata, the lenticels which traverse the periderm, and the peculiar 

 ventilating pores which occur in certain aerial roots. 



A. STOMATA. 



1. Structure and mechanism of stomata? 10 



The stomatic pores are the epidermal outlets of the ventilating 

 system. They are never surrounded by ordinary epidermal elements ; 

 on the contrary, each is enclosed by a pair of specialised guard-cells (in 

 Mosses the arrangement is slightly different), which constitute the 



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