ATMOSPHEnlC Ain AS THE POOD OP PLANTS. S? 



lower part in a hole in a glass cover,* that was cemented 

 air-tight upon the vessel. The stem itself was cemented 

 by soft wax into the liole, so that the interior of the ves- 

 sel was completely cut off ffom direct communication with 

 the external atmosphere. The plant thus situated had its 

 roots in an atmospliere as neaily as possible saturated with 

 vapor of water, while its leaves were exposed to the ex- 

 ternal air. After four days had expired, the entire appa- 

 ratus, plant included, had lost 1.823 grm. Thereupon the 

 plant was removed from the vessel and weighed by itself; 

 it had lost 2.188 grm. The loss of the entire apparatus 

 was due to vapor of water, which had escaped through 

 the leaves. The diiference between this loss and the los? 

 which the plant had experienced could be attributed only 

 to an exhalation of water through the roots, and amount 

 ed to (2.188 — 1.823=) 0.365 grm. 



This exhalation of water into the confined and moist at- 

 mosphere of the glass vessel is explained, according to 

 Sachs, by the fact that the chemical changes proceeding 

 within the plant elevate its temperature above that of the 

 surrounding atmosphere. 



Knop, in experiments on the transpiration of plants, 

 {Y. St., VI, 25.5,) obtained similar results. He found, 

 however, that a moist piece of paper or wood also lost 

 weight wlien kept for some time in a confined space over 

 water. He therefore concludes that it is nearly impossible in 

 the conditions of such experiments to maintain the air sat- 

 urated with vapor, and that the loss of weight by the roots 

 is due, not to the heat arising from internal chemical 

 changes, but to simple evaporation from their surface. In 

 one instance he found that a portulacca standing over 

 night in a bell-glass with moistened sides, did not lose, but 

 gained weight, some dew having gathered on its foliage. 



* The cover consisted of two semicircular pieces of ground glass, each of 

 which had a small semicircular notch, so that the twoconld be brought togethei 

 by their straight edges around the stem. 



