5O2 J. ARTHUR HARRIS 



the Piperaceae. The number under each of the averages is the number 

 of species, not the number of determinations, upon which it is based. 



The averages for terrestrial herbaceous species are those already 

 published (Harris and Lawrence, 19170). 



The general mean for the region has been computed by averaging 

 the species means for the individual habitats. Thus if a species 

 occurs in both the Leeward Ravines and the Ridge Forest it is counted 

 twice, whereas the species which occur in one of these habitats only 

 will be counted but once. Thus the numbers of the species given for 

 all habitats is the number of species weighted with the number of the 

 sub-habitats in which they occur. 6 



The comparison between the epiphytic and the terrestrial her 

 baceous forms has been made in two ways. First, the actual differ 

 ences in the average depression of the freezing point and in the average 

 calculated osmotic concentration have been determined and are given 

 with their signs. Second, the average values of P of the epiphytes 

 have been expressed as a percentage of the value for terrestrial herbs. 7 



An examination of the nine comparisons between the epiphytic and 

 terrestrial herbs of the four individual habitats shows that the con 

 centration is in every instance lower for the epiphytic forms. The 

 averages are roughly 4.0 to 5.4 atmospheres lower in the Orchidaceae, 

 3.8 to 5.2 atmospheres lower in the Bromeliaceae, and 3.4 to 3.6 atmo 

 spheres lower in Peperomia of the Piperaceae. 8 



There now remains for consideration only the half shrubby ges- 

 neraceous epiphyte Columnea hirsuta. One determination from the 

 Leeward Ravines gives A = 0.395, P = 4-76. Eight constants from 

 the Windward Slopes and Ravine average A = 0.354, P = 4-28. If 

 these be compared with the averages for herbaceous vegetation from 

 the same habitats, differences in P of 2.83 for the Leeward Ravine 

 determination and of 3.24 for the Windward habitats are secured. 



6 This method of computing the average has both advantages and disadvantages. 

 For present purposes it is quite adequate. 



7 Practically the same percentages are secured by using the average values of 

 freezing-point lowering, but since the relationship between A and P is not strictly 

 linear the results are not exactly identical. 



8 Comparisons with the herbaceous plants of the regions as a whole show a 

 concentration 5.1 atmospheres lower for Orchidaceae, 4.5 atmospheres lower for 

 Bromeliaceae, and 4.2 atmospheres lower for Peperomia of the Piperaceae. The 

 averages for the whole region is obtained by weighting those of the individual habitats 

 with the number of species examined. 



