RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 31 



funnel (Haberlandt) broad at one end and tapering at the other. 

 Tissues 



chlorenchym the green tissue of the leaf, *. e. mesophyll. 

 compact with infrequent intercellular spaces, 

 close with few small intercellular spaces (smaller than the cells), 

 loose with frequent small intercellular spaces, 

 lacunose with frequent large intercellular spaces (as large as or 

 larger than the cells). 



HYDROPHYTIC TYPES 



Alpine lakes: light ; available water 100% ; humidity 100% ; 

 temperature 4O-65. 



ISOETES LACUSTRIS PAUPERCULA : spongophyll 750 //, in diameter; 

 epidermis 15 /u,; cuticle thin; chlorenchym of uniform globose cells 

 compactly arranged and surrounding in each quarter-section an air- 

 passage with diaphragms of 5-6 lobed star-shaped cells. (Plate 



I, fig. i.) 



Subalpine brook bank : light 0.03 ; available water 25-30% ; 

 humidity 60-85% I temperature 47 -69. 



SAXIFRAGA PUNCTATA : diphotophyll 450 ju, ; epidermis, upper 30 /*, 

 lower 20 /*, ; cuticle thin ; chlorenchym (400 ju.) I row prolate pal- 

 isade cells 70 /A ; a region of loose subglobose and irregular cells 

 merging into large elongated cells and large air-spaces. (Plate 



II, fig. i.) 



Since most of the hydrophytes secured had both submerged and 

 aerial leaves, or else aquatic and amphibious forms, they have been 

 discussed under " Polydemic Species." Of the two here mentioned, 

 Isoetes is typically a submerged plant. The physiological processes 

 of submerged leaves are of the simplest sort, since the assimilative 

 cells are in direct contact with their food supply. This fact, to- 

 gether with that of reduced transpiration and diffuse light, accounts 

 for the undifferentiated structure of the chlorenchym. Saxifraga 

 punctata is unusual in being an amphibious plant which grows in the 

 shade. The effect of the reduced light is seen in the small amount 

 of palisade tissue, which comprises but 17 per cent, of the chloren- 

 chym, while the presence of abundant water and great humidity is 

 apparent in the character of the sponge tissue. Adaptations to 

 prevent the stagnation of the sap current are necessary under these 

 conditions. This is attained in the great increase of transpiring 



