436 VENTILATING SYSTEM 



been noted by Luerssen as characteristic of the parenchymatous tissues 

 of many Fern-petioles, are likewise composed of pectic substances. 208 



B. THE STRUCTURE OF VENTILATING SPACES AND VENTILATING 

 TISSUES, CONSIDERED IN RELATION TO CERTAIN PHYSIOLOGICAL 

 FUNCTIONS. 



1. The ventilating system in relation to respiration. 



The relations between the structure of the ventilating system and 

 the function of respiration, are naturally most clea 1 1 f. revealed in the 

 case of organs which are altogether colourless, or which at any rate 

 contain very little chlorophyll, because gas-exchange is, in these cir- 

 cumstances, to a very large extent respiratory in character. Organs 

 which respire actively, are invariably provided with a well-developed 

 ventilating system ; but it would not be correct to assume that there 

 is always a close correspondence between the average intensity of 

 respiration and the degree of development of the ventilating 

 spaces. 



In young organs which are growing rapidly, and hence also respir- 

 ing actively, air-containing intercellular spaces generally make their 

 appearance long before any appreciable tissue-differentiation takes place. 

 In some instances especially among roots ventilating passages extend 

 far into the primordial meristem of the apical region (Fig. 18). Occa- 

 sionally the respiratory requirements of young organs are so great, as 

 to necessitate the formation of special ventilating organs. An interest- 

 ing illustration is furnished by the young leaves of Nephrodium 

 stipellatum, Hook., a Fern which is not uncommon in moist wooded 

 ravines in Western Java. 209 As long as they remain rolled up, the young 

 leaves of this Fern are enveloped in a coating of mucilage, several mm. 

 in thickness, which is secreted by special mucilage-hairs, and in all 

 probability acts as a protective covering. This thick layer of muci- 

 lage would seriously impede respiration, were it not perforated, at 

 frequent intervals, by certain conical or awl-shaped structures, which 

 represent special breathing organs or pneumatophores. In the upper 

 part of the leaf, these pneumatophores, which may reach a length of 

 5 mm., arise close beside the points of attachment of the pinnae, while 

 further down, on the' petiole proper, they are arranged in two longi- 

 tudinal rows, and are, also, provided with shield-shaped appendages of 

 unknown significance. The whitish external appearance of these 

 pneumatophores at once suggests that they possess a spongy texture ; 

 as a matter of fact, they consist of a loose aerenchyma traversed by 

 large ventilating spaces, which communicate with the outer atmosphere 

 by means of numerous stomata. The walls of the aerenchyma-cells 



