52 HORTICULTURAL SOCIETY OF NEW YORK. 



structure of leaf from that exhibited under less strenuous con- 

 ditions of evaporation. 



A consideration of evaporation as a controlling factor in 

 plant growth would be logically incomplete without mention of 

 the abnormal behavior of many pronounced xerophytes when 

 subjected to conditions of low evaporation rate. Among gar- 

 deners the succulents are generally regarded as very difficult to 

 grow in the more humid regions, and especially in glass houses. 

 Such plants often fail to develop normally and often meet their 

 death through the action of fungus diseases of the damping-off 

 form. It is possible for the gardener to provide conditions of 

 soil moisture very nearly approximating those of arid regions, 

 but in most conservatories it is practically impossible to attain 

 anything like the atmospheric conditions to which desert suc- 

 culents are normally subjected for a great part of the year. I 

 have had occasion to study the fine collection of cacti grown 

 under glass at the Missouri Botanical Garden, at which place I 

 enjoyed the privileges of working in the conservatories during 

 two months of last winter, and it appeared that while many spe- 

 cies were thriving well under the artificial conditions, yet a num- 

 ber of forms were not at all healthy. It was further deter- 

 mined that these plants were not transpiring in the normal man- 

 ner. I have also often observed at the Desert Laboratory that 

 pot-grown cactus seedlings are very prone to die of a damping- 

 off disease or root disease, especially when subjected to a low 

 evaporation rate. It would thus seem that certain of these 

 plants which are adapted to great evaporating power of the air, 

 do not thrive when exposed to low evaporation. Perhaps the 

 transpiration stream is necessary in such cases. It may be that 

 the epidermal covering does not develop normally in the absence 

 of rapid transpiration, and that this explains the frequent de- 

 struction of these succulents by fungi when grown in conser- 

 vatories. 



It is possible that evaporation plays an important part in 

 causing the marked differences in the vegetation of neighboring 

 sunny and shady areas. In an experiment carried on by the 

 Desert Laboratory in cooperation with a large number of workers 

 distributed over the United States, a number of tests of the 



