33 



ceive that this appearance is caused by very large and deep hol- 

 lows in the cell walls, distributed in such a manner that tlie de- 

 pressions in the wall of one cell correspond to elevations or pro- 

 jections in that of the contiguous one, and vice versa, (Fig. 4); 

 besides, deep pits are found abundantly in the thick walls of these 

 cells. The palisade tissue underneath the epidermis is composed 

 of two or three, sometimes even four tiers of cylindrical, narrow 

 cells, with numerous air-spaces between them, and containing, on 

 their vertical walls, large chlorophyll grains. The air cavities 

 under the breathing pores are very large and deep, on account 

 of the great height of the epidermis and palisade layers. The 

 arrangement of the palisade cells into distinct groups is very strik- 

 ing. The upper layer of each group is joined to a group of epi- 

 dermis cells (which is determined by the position of the surround- 

 ing stomata or, rather, their air cavities), while the lowest tier 

 connects with some large "collecting cell" of the spongy tissue 

 or of the conducting bundle. 



The leaf structure is greatly modified in the central zone. 

 The absence of stomata mentioned above causes the absence of the 

 corresponding air-cavities, and the paHsade tissue is reduced to 

 one or two layers of cells; in the very center there are no typi- 

 cal palisade cells at all. Besides, the epidermis cells of this zone 

 have almost perfectly straight walls which do not bulge outward 

 and inward. This seems to justify the assumption that the pccu- 

 liar structure of the epidermis cells has something to do with the 

 breathing process. Supposing that the undulating walls of these 

 cells could be straightened out, or that the elevations could even 

 be changed into depressions and reversely, by changes in the 

 turgor or the atmospheric pressure, we could easily infer that, ac- 

 cording to circumstances, either a powerful suction or pressure 

 would be produced. Such pressure seems certainly to be nec- 

 essary to keep all the numerous large passages in leaf, stem and 

 root filled with air in order to resist the great pressure of the 

 surrounding water, and to renew the same constantly, so that the 

 chlorophyll, which is met with almost all over the plant, may do 

 its work of assimilation. 



In connection with these considerations, I might mention that 

 such contrivances as are supposed to assist the plant organs in 



