GENERAL CONCLUSIONS. 109 



each set of plants exists, under its own climate, the desert plant loses 

 far more water in transpiration per unit area than does the plant of 

 the rain-forest. 



In the herbaceous plants of the rain-forest there is no correlation of 

 stomatal openness and relative transpiration rate, at least during the 

 morning and mid-day hours. These plants possess extremely thin 

 epidermal structures, through which the loss of water in transpiration 

 is found to be slightly greater than the loss through the stomata. The 

 preponderance of cuticular transpiration is largely responsible for the 

 fact that the total transpiration is extremely sensitive to the prevailing 

 evaporation conditions and is partially responsible for the fact that 

 the relative transpiration rate of these plants when placed in darkness 

 is not lower than their rates in the light. 



The writer's interest in the behavior of rain-forest plants has centered 

 in the most hygrophilous forms, but these must not be taken as typi- 

 fying the vegetation as a whole. The difference between the climate 

 in the interior of the forest and in openings in the forest and the dif- 

 ference between the climate at the floor of the forest and in its canopy 

 are as great as the normal difference between widely separated places. 

 Corresponding with these differences of climate are striking differences 

 in the character of the vegetation, both when the forest floor is con- 

 trasted with cleared thickets and when it is compared with the forest 

 canopy. The dominant trees of the best developed rain-forest possess 

 very sclerophyllous foliage; the high epiphytes have coriaceous succu- 

 lent leaves ; ^elow them are to be found the normal leaves of the larger 

 shrubs; beneath these the thin leaves of the larger herbaceous plants 

 with an open mesophyll of several layers of cells; while in the lowest 

 and most shaded situations are to be found such small plants as Pepe- 

 romia pellucida, with a single layer of mesophyll cells, and the filmy 

 ferns, with leaves which are a single layer of cells in thickness. This 

 tremendous contrast between the members of the several layers of the 

 rain-forest and the vertical differences of climate to which the contrast 

 is chiefly due are both dependent upon the existence of the forest itself 

 and the power which each stratum of vegetation has for the mainte- 

 nance of the conditions which are vital to the plants of the next stratum 

 below. The dominant trees and the high epiphytes are capable of 

 withstanding the water loss to which they are subjected in the infre- 

 quent periods of cloudless weather, without fog or rain and with abnor- 

 mally low humidity ; while the hygrophilous plants of the lowest stratum 

 are protected from the full duration of the dry periods by the shade in 

 which they are growing and by the slowness with which the enormous 

 quantities of moisture are given up by the soil, the rotting logs, the 

 beds of mosses and hepatics, and the litter of fallen twigs and leaves. 



There is no type of vegetation in which may be found a wider 

 diversity of life forms than exist side by side or one above the other in a 



