AIR-SPACES 435 



against several passages, which run parallel to the long axis of the cell, 

 communicating with one another at the surfaces of contact between 

 successive layers of palisade-cells. Where a number of elongated 

 cells are arranged in longitudinal series, it is usual to find air-passages 

 of great length extending along the edges of the cells. This arrange- 

 ment prevails, for example, in the leaves of Elodea canadensis, 

 Galanthus nivalis and Leucojum aestimim, and in the inner region of 

 the primary cortex of many roots. The wide air-passages which 

 traverse all the vegetative organs in the case of many marsh- and 

 water-plants, may be regarded as elongated passages of this type, 

 which have become greatly distended. 



In the spongy parenchyma of foliage-leaves, the ventilating system 

 shows a marked tendeucy to assume the form of inter-communicating 

 cavities or chambers, owing to the fact that the cells are usually 

 produced into a number of arms or branches, which only abut against 

 one another at their distal ends. Large polyhedral air-chambers, such 

 as are found in the leaves of Pistia and Pontederia, and in the fronds 

 of Lcmna, may be compared to the wide air-passages included in the 

 preceding class of ventilating spaces. 



Ventilating clefts may arise between individual cells, just like air- 

 passages or -cavities, or they may separate whole layers or sheets of 

 cells from one another. The former condition is exemplified by the 

 leaves of many Myrtaceae, further by species of Scirpus and by 

 Cladium Mariscus ; the tubular photo-synthetic cells in the leaves of 

 the last-mentioned plant are almost entirely separated from one 

 another by such intercellular clefts ; only a few small circular areas 

 of the walls remain in contact with one another. Larger ventilating 

 intercellular clefts are particularly prevalent in photosynthetic tissues ; 

 they occur, for example, in the leaves of Pinus, Abies and Cryptomeria, 

 and also in many Monocotyledons. 



The ventilating spaces in a particular tissue, whether developed as 

 passages, chambers, or clefts, frequently present the appearance of 

 isolated cavities. Nevertheless, the different spaces are, in the vast 

 majority of cases, connected in such a manner that a free circulation of 

 gases can take place throughout the tissue. The connecting channels 

 are often exceedingly small, and hence may easily be overlooked. 



The cell-walls which abut against intercellular spaces, generally 

 remain unthickened. In close proximity to pneumathodes and 

 especially beneath stomata they may be covered by a cuticle. Much 

 more generally, however, intercellular spaces are lined by a delicate 

 pellicle of pectic material, owing to the fact that the spaces arise by 

 the splitting apart of the middle lamellae : the tubercles and rods, pro- 

 jecting into the spaces and traversing their entire width, which have 



