107 
The quantity of water so used is as variable as the amount of precipitation, 
and, in fact, within certain limits depends largely upon it. That is to say, a plant 
will transpire in proportion to the amount of water which is at its disposal. Trans- 
piration is also dependent on the stage of development of the plant, on the 
nature of its leaves, and the amount of its foliage, on temperature, humidity, and 
circulation of the air, on the intensity of the sunlight, and on temperature and 
structure of the soil and other meteorological conditions. Eain and dew reduce the 
transpiration, wind increases it. The amount of transpiration depends considerably 
on the thickness of the leaves, therefore the surface of the foliage is not a reliable 
measure, but should be compared with the weight. With so many factors to vary 
them, the values which may be given for the amount of transpiration of various 
kinds of trees can only be approximations of its range within wide limits. 
(Harrington, op. cii.) 
All vegetation takes up a certain amount of water, a part of which is con- 
sumed in building up its body, and a still larger part returned to the atmosphere by 
transpiration during the growth. 
The factor of dissipation having being fully discussed, it need not be further 
considered here, except to recall the conclusion that forest growth transpires con- 
siderably less than other kinds of vegetation. 
Since this water is given off again to the atmosphere in the locality where 
it has fallen thus enriching the atmospheric moisture and is, therefore, only 
diverted temporarily for the purpose of doing duty in producing useful substance 
and retaining it in the locality \vhere it has fallen for a longer time, transpiration 
may even be considered as an element of conservation. 
There is still to be considered a certain amount of moisture which is retained 
and stored up in .the body of the plant, partly as a necessary permanent constituent, 
partly as a temporary constituent, being evaporated when the plant dies or the 
wood is seasoned. The amounts thus retained vary considerably according to age, 
capacity for transpiration, site, soil, climate, density, slow or rapid growth, weather, 
seasons, and even the time of day. It is, therefore, almost impossible to give 
anything but very rough approximations, especially as also the different parts of the 
tree vary considerably in the amounts of water present. (Fernow, op. cit.) 
8. SOME USES OF FORESTS. 
(a) To temper Floods. 
I will deal with this subject in another paper, on " The Mitigation of Floods 
by Forestry Operations," hence I propose to give more cursory treatment in this 
place than its importance demands. I think very few men will dispute the use of 
forests in mitigating the effects of downpours of rain in regard to the flow of 
watercourses. 
