THE FOREST AND THE PRAIRIE. 251 



the leaves. Water taken in through the roots is united with carbon 

 dioxid absorbed from the air through the leaves. In the presence of 

 light, by an unknown chemical process, the carbon dioxid and water are 

 changed to a carbohydrate and oxygen is given off. Of course, light 

 reaches the trees only in the day time. This process needs a certain 

 amount of heat, but small quantities of starch can be produced by trees 

 with evergreen leaves even at temperatures slightly below freezing. 

 There are probably many warm days dunng the non-growing season when 

 considerable quantities of carbohydrates like starch and sguar can be 

 made. Especially would this be true in the early spring and late fall. 

 The trees with broaa leaves can manufacture more food during the sum- 

 mer months than trees with needle leaves, for they have more surface ex- 

 posed to light, and the greater the green surface, other things being equal, 

 the greater the amount of food that can be formed. But the time for 

 the making of food by these broad leaved plants is practically limited to 

 the time of the year during which they have their leaves. As soon as 

 the leaves are shed little or no food can be produced. Thus, while they 

 have an advantage over the evergreen trees during the summer, the latter, 

 because they can work more or less during the whole year, may manu- 

 facture more food in a year than the former. This is more likely to l^e 

 the case in high latitudes where the temperature of the summer months 

 is comparatively low, than in more southerly climates. Especially is it 

 the case in climates with a more equable distribution of the heat through- 

 out the year, the summer months having a comparatively low mean and 

 the winter months a comparatively high mean. Such is the climate in 

 the Puget Sound regon, where conifers are developed best in America. 

 In the Flathead valley the distribution of the temperature throughout the 

 year is not so equable as it is in the Puget Sound region, but it is more 

 so than in a similar latitude in the eastern part of the United States. 

 The evergreen trees reach a more luxuriant development in this valley 

 than anywhere in the east, but of course are not so luxuriant as in the 

 Puget Sound region. 



The greatest danger that trees have to meet is an excessive loss of 

 water. They are excluded from those climates that have little rain fall. 

 A prairie vegetation can exist in these places because the plants growing 

 here have met the danger of drought better than trees. There may be 

 climates where the rainfall is so little that even prairie plants cannot 

 thrive; then a desert is the result. There is, however, no true desert 

 region in Montana, although in places it approaches the desert condition. 

 These regions are known as the great plains. In such regions the sage 

 brush is a characteristic plant, although it is not so conspicuous an ele- 

 ment there as in the more desert like regions. 



When the clmate of a country makes conditions favorable for a cer- 

 tain form of plant life, then that form gives character to the landscapt. 

 If the grass and its associates give character to the region, then there is 

 a prairie formation. If trees give a tone to the landscape, then there is 

 a forest formation. 



It may not l)e out of place to compare the relative capacity of these 

 two forms of plants (the grass form and the tree form), to meet this 



