44 



A TEXTBOOK OF BOTANY [Ch. Ill, 6 



cease transpiration, though its amount may be insignificant, 

 while it ranges all the way up to above 250 grams per 

 square meter of leaf area per hour. The conventional con- 

 stant (page 25) for 

 greenhouse plants 

 is 50 grams per 

 square meter per 

 hour by day, and 

 10 by night, or 

 30 night and day 

 together, or 720 

 grams per 24 hours. 

 This amounts to 

 108,000 grams per 

 season, which 

 equals a layer of 

 liquid water all 

 over the leaf some- 

 what more than a 

 decimeter deep; 

 and presumably 

 this figure will 

 prove higher for 

 plants out of doors 

 in the summer. If 

 one can see the 720 

 grams transpired 

 in 24 hours stand- 

 ing in a measuring 

 glass in the center 

 of a square meter 

 of surface, he will 

 realize better the most striking fact about transpiration, — 

 its remarkably large amount. All of this water, it must be 

 remembered, has to be absorbed by the roots from the soil, 

 and lifted through the stem. 



Fig. 18. — A conclusive demonstration of trans- 

 piration ; X |. The bell jar was dry when placed 

 over the plant. Its bottom is a plate split and 

 perforated in such a way as to fit closely around 

 the stem of the plant. 



