PNEUMATOPHORES 437 



are covered externally with innumerable thickenings in the shape 

 of tubercles or short rods, which project into the intercellular 

 spaces. 



There can be little or no doubt, that these remarkable structures 

 actually represent special organs of ventilation, since their air-spaces 

 communicate freely with the ventilating system of the rhachis and 

 petiole. 



It is probable, that a similar respiratory function should be attributed 

 to the so-called fore-runner-tip ( Vorlauferspitze) of leaves, a structure 

 first observed by Criiger, and more recently investigated in detail by 

 Kaciborski. 210 This is a highly characteristic feature of the very young 

 leaf in many lianes, and in certain other tropical plants ; it takes the 

 form of a long narrow terminal prolongation of the lamina, being 

 frequently marked off from the blade proper by a slight constriction. 

 The fact that it bears an unusually large number of stomata, suggests 

 that it carries on gaseous interchange, and particularly the respiratory 

 absorption of oxygen and evolution of carbon dioxide, on behalf of the 

 leaf-blade, while the latter is still in the embryonic condition. In 

 certain cases it seems to be more immediately concerned with the 

 secretion of (liquid) water. For the sake of convenience, the aeren- 

 chymaof the so-called "breathing-roots" which occur in many marsh- 

 and water-plants, will be discussed in a later section of this chapter. 



2. The ventilating system in relation to photosynthesis. 



The photosynthetic organs of land-plants are almost always covered 

 by a typical epidermis. Hence the characteristic gas-exchange of the 

 photosynthetic system, that is, the absorption of carbon dioxide and the 

 liberation of oxygen, is carried on almost entirely through the mediation 

 of the ventilating system. Even those palisade-cells which abut 

 directly against the epidermis, obtain their carbon dioxide from the 

 adjoining intercellular spaces, and not from the outer atmosphere vid 

 the epidermal cells. It is for this reason that every photosynthetic 

 cell borders upon intercellular spaces at one or more points. In the 

 case of palisade-cells, it is usually strips of the lateral walls of varying 

 breadth which thus come into direct contact with air-spaces ; it will 

 be recollected, that the chloroplasts tend to congregate on these very 

 strips, while they are often altogether absent from the cross-walls. 

 The fact that chloroplasts favour those walls which abut against inter- 

 cellular spaces, may be just as readily verified in subepidermal palisade- 

 cells as in more deep-seated layers. It furnishes histological evidence 

 in support of the conclusions drawn from experimental data. Stahl 

 and Blackman 211 both conclude, on independent experimental grounds, 

 that, under normal conditions, the photosynthetic tissues are entirely 



