EXTERNAL COMMUNICATIONS OF THE AERIFEROUS SYSTEM 195 



hydrostatic pressure in the guard-cells is more marked than that of the other 

 epidermal cells, but Pfeffer could find no perceptible difference between them in 

 Amaryllis formosissima and Tradescantia discolor. 



Turgidity and its regulation are in all cases dependent upon the amount 

 of material available for osmotic purposes. Hence the formation of organic 

 substance by the assimilatory activity of the chloroplastids may play a prominent 

 part in inducing the movements of the guard-cells. Nevertheless, the move- 

 ments caused by light are not due solely to the accumulation of soluble as- 

 similatory products, as Mohl (I.e., p. 717) and Kohl (I.e., p. 39) suppose, for in 

 well-nourished plants light causes the stomata to open even in an atmosphere free 

 from carbon dioxide. Light acts here as a stimulus causing movement, just as it 

 does in other movements induced by stimuli. It is only owing to the insufficient 

 development of the specific reactive power that stomata with non-chlorophyllous 

 guard-cells 1 , and indeed a few in which chlorophyll is present, sometimes do not 

 perceptibly react to a change in the illumination. 



It is not, however, surprising that the guard-cells should react more energetically 

 after previous exposure to light (Mohl, 1. c., p. 7 1 6), or that they should be independent 

 of the accumulation of assimilated products; external influences, such as light, 

 frequently exercise inductive actions of a tonic character which persist for a time 

 (Leitgeb, I.e., p. 129). When a stoma is abnormally wide open the amount of in- 

 soluble reserve-food-material in the guard-cells decreases, but this merely corresponds 

 with the usual phenomena of turgor regulation. 



The opening of the stomata in every case increases the rapidity of 

 gaseous exchange, which is, however, dependent upon other circumstances 

 as well, such as the length of the capillary channels and the position of the 

 stomata. In sunken stomata, and especially when the channel leading to 

 the pore passage is narrow, gaseous diffusion is rendered more difficult. 

 The stomata are always, it is true, exceedingly narrow openings, since the 

 diameter of the pore passage rarely surpasses 0-03 mm., and its area 

 0-0046 sq. mm. 2 



This is, however, compensated by their great abundance, for com- 

 monly 100 to 300 stomata may be present on i sq. mm. of leaf surface, and 

 in a few cases as many as 700. Taken as a whole, the gaseous exchanges 

 through the lenticels are less in amount, for these are very much fewer in 

 number, although a lenticel transmits more gas through its numerous inter- 

 cellular canals than does a single stomatal pore. Moreover, even in young 



1 Kohl, I.e., p. 39. The guard-cells in etiolated plants may, according to Leitgeb (I.e., p. 175), 

 be widely open. [It must, however, be remembered that etiolated chloroplastids are capable of weak 

 CO 2 -assimilation in the absence of all chloroprn 11 (cf. Sect. 52). In leaves in which the chloroplastids 

 of the guard-cells had been bleached or rendered permanently inactive by the prolonged action of 

 intense light (Ewart, Ann. of Bot., 1897, Vol. xi, p. 475), the stomata were found in all cases to be 

 closed, but this might be simply owing to the guard-cells having been injured, for when the exposure 

 is intense and prolonged, the death of the entire cell soon follows that of the chloroplastids.] 



2 For anatomical details, see de Bary, Comp. Anat., 1877, p. 39; Mohl, 1. c. ; Weiss, Jahrb. f. 

 wiss. Bot., 1865-6, Bd. iv, p. 181, &c. 



O 2 



