434 VENTILATING SYSTEM 



In the case of organs that are permanently submerged, or that 

 grow in very wet soil, the possibility of gaseous interchange may be 

 entirely dependent upon the continuity of their air-spaces with the 

 ventilating system of the portions which are exposed to the outer atmo- 

 sphere and provided with pneumathodes. Such organs may, on the other 

 hand, carry on a direct exchange of gaseous material with the surround- 

 ing medium ; but in this case the process is so slow, that the develop- 

 ment of large internal air-reservoirs in the submerged or subterranean 

 structures becomes an absolute necessity. 



As a rule the ventilating system is entirely composed of inter- 

 cellular air-spaces ; in accordance with its function, it is a continuous 

 system, and permeates all the other tissue-systems that have to 

 carry out gaseous interchanges for purposes of respiration, or in 

 connection with some special function. Another general feature of 

 the ventilating system is the development of external outlets or 

 pneumathodes, which exhibit great variety as regards the details of 

 their structure. In the majority of cases, each individual tissue the 

 photosynthetic tissue, for example produces the ventilating spaces 

 which it needs by a partial separation of its cells. In certain 

 instances, however, especially where large air-reservoirs are required, 

 the formation of intercellular air-spaces becomes the sole, or at 

 any rate, the principal function of a special tissue, which may 

 consequently be termed ventilating tissue or aerenchyma. 



II VENTILATING SPACES AND VENTILATING TISSUES. 



A. FORM OF THE AIR-SPACES AND CHARACTER OF THEIR WALLS. 



If the ventilating spaces of the plant-body are to perform their 

 functions satisfactorily, it is above all things essential that they should 

 form a continuous system. The general arrangement of this system is 

 such, that every cell of the tissue which it ventilates is actually in 

 contact with one or more intercellular air-spaces. In other respects 

 the extent and distribution of the ventilating spaces vary greatly, 

 according to the requirements and special functions of the different 

 tissue-systems. 



Generally speaking, ventilating spaces may take the form 

 of elongated passages, of wide cavities, or of narrow clefts and 

 interstices. In ordinary parenchymatous tissues, composed of iso- 

 diametric cells, the intercellular spaces are developed as narrow 

 passages, extending along the rounded-off edges of the cells, and 

 collectively forming a dense mesh work which spreads out equally in 

 all directions. In the case of palisade-tissue, every cell abuts directly 



