igip] CRIBBS—TILIA AMERICANA 281 



general shape, and texture of the leaves at station E are also dis- 

 tinctly xerophytic, but it has been noted that notwithstanding 

 this the transpiration index is higher, a fact not at all in accord- 

 ance with the behavior of desert plants so far reported, for they 

 are characterized by a low transpiration index, as pointed out by 

 several workers. With this fact in view Bakke (i) has suggested 

 the foUar index of relative transpiring power as a test for the 

 mesophytism of a plant, as follows: "As a result of the prehminary 

 study, it may be suggested that plants exhibiting a diurnal foHar 

 transpiring power of less than o . 30 may be regarded as xerophytes, 

 while those exhibiting indices above o . 70 may be considered mes- 

 ophytes." It may be seen aL once that the behavior in Tilia is 

 the reverse of that common to desert plants, and hence an appli- 

 cation of this test would lead to confusion. I believe with Bakke, 

 however, that with certain reserve this method may be used as a 

 fair indicator of mesophytism, provided two precautionary meas- 

 ures are taken: first, that the species under consideration be 

 chosen in its normal environment and not in an abnormal or 

 forced one; and second, that hourly readings be taken for at 

 least two full days and that the relative humidity and tempera- 

 ture conditions be carefully employed in calculating the results. 

 This latter precaution is necessary because of the great variation 

 in the index at different times of the day, in the first place, and 

 because of the wide variation of the indices at any particular hour 

 on two successive days when the relative humidity or temperature 

 has undergone considerable change. 



The development of a xerophytic leaf under unusual condi- 

 tions of exposure has been found in Tilia to result in a leaf less 

 effective in preventing water loss than are desert types. This 

 may be attributed to a lagging of the effect behind the causal 

 factors. On the other hand, such lagging may be considered as 

 not occurring, for there is always a favorable balance in water 

 relations which permits a greater vegetative activity than would 

 be possible in desert plants, and becomes possible on the open 

 sands only because of a sufficient and permanent growth water 

 throughout the growing season. 



