TISSUES AND SIMPLE ORGANS. 133 



egress of air. Such anatomical adaptation's we find in the stomata 

 (breathing-pores) of the epidermis and in the lenticels of the cork- 

 tissue. 



It will not be found difficult to understand the structural dif- 

 ferences between land-plants and water-plants. The supply of air 

 in submerged water-plants is very limited and can be obtained only 

 from the water (absorbed air). Plants temporarily or permanently 

 partially submerged are also limited in their supply of air as com- 

 pared with land-plants. Considerations lead us to the postulate 

 that water-plants must carry a supply of air with them. Compara- 

 tive observation reveals the following : Every palisade-cell of the 

 leaf of a land-plant which, for example, lies in contact with six 

 other cells is laterally surrounded by six delicate intercellular 

 canals corresponding to the six prismatic corners and edges of each 

 cell ; every cubical cell is enclosed by twelve minute canals, etc. 

 Otherwise, comparing the intercellular 

 spaces of land-plants with the air-spaces 

 occurring in the leaves and steins of 

 water-plants and marsh-plants, it is 

 noticeable to the naked eye that the 

 latter appear as cavities and channels. 

 These contain a large supply of air to 

 satisfy requirements. Such, in one re- 

 spect, is the interpretation of this phe- 

 nomenon. The relation between the 

 magnitude of intercellular spaces and the 

 water contained in the surrounding 

 medium was known to older anatomists. 

 The accompanying figure (73) shows a 

 fragment of the parenchymatous tissue 



of a water-plant (from the leaf of Acorns Calamus) magnified 

 about three hundred times.' A positive pressure, due to the proc- 

 ess of assimilation, 1 has been observed in the air-chambers of sub- 

 merged green water-plants. 



Moreover, the air contained in submerged and floating plant- 

 organs tends to reduce the specific gravity of the organs, thus en- 

 abling them to float, or at least decreasing the tendency to sink. 



PFEFFER, Physiologic, I. Band, p. 85, et seq. 



