32 PLANT STDDIES 



a second tumljler over the blade of the leai, whidi pi-ojects 

 above the cardboard (see Fig. 34). It will be observed tliat 

 moisture given off from the surface of tlie woi-kiiig leaf is 

 condensed on the inner surface of the inverted tiiiid)h'r. 

 The cardljoard is to shut off evaporation from tlie water 

 in tlie lower tuudiler. 



Wlieu the amount of water given oil by a single leaf is 

 noted. Some vague idea may be formed as to the amount oi 

 moisture given off Ijy a great mass of vegetation, such as a 

 meadou' or a forest. It is evident tliat green iihints at 

 worlv are contributing a very large a.mount of moisture to 

 the air iu the form of water vapor, moisture which lias Ijcen 

 absorl)ed 1)V some region of the plant. 'I'hc foliage leaf, 

 therefore, iiuiy lie regarded as an organ (if I mni<i)lration, 

 not that, tlie leaves alone arc engaged in ti'aiispiration, for 

 many parts of the plant do the same thing, )uit lieca.use tlio 

 foliage leaves are the chief seat of transpiration. 



The ini[)ortant fact in connection witli transpiration is 

 not that moisture is given otf liy a.ctive foliage leaves, 

 Ijiit that this escaping moisture is tlie external indication 

 of some work going on within the leaf. I'ranspiration, 

 therefore, may not lie regarded so mucli as work, as tlie 

 result, and lience the indication of work. In case the 

 leaves a.i'e submerged, as is true of many plants, it is evi- 

 dent that transpiration is practically idicidved, for the 

 leaves are alreadv Itathed with water, and umler such cir- 

 cumstances water vapor is not given otf. The same is true 

 of gi'een water plants without leaves (such as alg;v). It is 

 evident that iiiider such circumstances leaf work must be 

 carrie(l on without transpiration. 



Tl . Respiration. — Another kind of woi'k also ma\- be de- 

 tecled in the foliage leaf, but not so easilv desciMUed, In 

 fact it escapeil the attention of liotanists long al'ler they 

 hail discovered photosynthesis and I i'a,nspii-nt ion. It is work 

 that goes on so long a.s the leaf is ali\e. never ceasiiii;' dav 

 or night. The external indication of it is the absorption 



