252 HUI.I.KTIX I'XIVKRSITY OF MONTANA 



dangei- of too great a loss of moisture. The danger to plants becomes 

 greatest during the dry summer months and during the winter. As 

 already shown, during the winter months the cold soil is not favorable to 

 absorption, and if the plant be subjected to evaporation then, it is likely 

 to transpire more than it can aljsorb. A cold soil then acts in a measure 

 like a dry one, and the plant is in danger of excessive loss of water during 

 the winter as well as during the dry summer months. 



The herbaceous plants meet the danger of transpiration and freezing 

 by dying down to the ground during the unfavorable season. The an- 

 nuals of course are tided over this season in the form of seeds, and the 

 perennials, beside seeds, have also underground stems. The latter, be- 

 cause they are protected by the soil, are in little danger of drying out. 

 The woody plants, on the other hand, are exposed to all the extremes of 

 winter. All trees are protected by their bark. In the young twigs this 

 Is not so thick, and hence these parts are more exposed than the older 

 parts with heavy bark. So it is with the seedlings; for, although not so 

 exposed as the older trees, the seedling stage of a tree is the most deli- 

 cate one in its existence. If the dry season comes on before its root 

 system is well developed, the seedling is very likely to perish. The seed- 

 ling is subject to other dangers that will be mentioned in another connec- 

 tion. Those trees that shed their leaves are not so exposed during the un- 

 favorable season as those that have their leaves the year round. Even 

 the trees without leaves may be winter-killed. On the whole, then, her- 

 baceous plants are less exposed during the dry and non-growing season 

 than the trees. 



Fires are detrimental to all vegetation, so it will not be out of place 

 to compare the two forms with regard to their powers in resisting fires. 

 A fire sweeping across the prairie may burn the surface clean of vegeta- 

 tion, but seldom if ever injures the underground parts. As soon as the 

 season permits, the grass will spring up from these underground stems. 

 Trees that have the power of developing suckers from roots will be 

 protected in the same way. But few conifers have this power. How- 

 ever, the bark of old trees is often a protection against fires. The bull 

 pine, Douglas spruce and western larch have thick bark and are pro- 

 tected in this way from fires that are not too intense. Young trees, how- 

 ever, succumb even to light surface fires, so acres of young trees may 

 often be destroyed by fires that will not injure the older trees. The per- 

 ennial herbaceous plant, then, has greater powers of surviving fires than 

 trees. 



Again, grazing animals are injurious to vegetation. The perennial 

 grass is protected, because, when the top is eaten off, the under ground 

 part is usually uninjured. Of course, if the above-ground green parts 

 are eaten off so frequently that they have no opportunity to manufacture 

 new food, the plant will be killed, and when the food stored in the imder- 

 ground stems is used up, then the plant will starve. Hence close pas- 

 turage is dangerous to the grass form. However, other forms of her- 

 baceous plants that have an objectionable taste or are covered by spines 

 or prickles will be avoided altogether by the grazing animals, or eaten 

 only when nothing else is available. Sheep are the most destructive anl- 



