26 THE PLANT LIFE OF MARYLAND 



growth offset the favorable climatic conditions of the region as a 

 whole. 



The forests of Maryland differ from place to place in the tree 

 species of which they are made up or in the relative proportion of the 

 component species. Such differences are of two kinds, — (a) the dif- 

 ference between forests in widely separated sections of the state, 

 which are due to the occurrence of tree species in the one section 

 which do not range into the other, and (b) the difference between 

 forest growths which are near together but occupy situations which 

 are different in soil, in the amount of soil water, etc. The first of 

 these differences in types of forest is floristic and has its basis in 

 those historical and climatic causes to which allusion has already 

 been made ; the second is an ecological difference, and is due to local 

 conditions only. A study of the forest types of an area so small as 

 to have a uniform flora and climate, for example one of the counties 

 of Maryland, brings out the importance of the aforesaid local condi- 

 tions. Of these the most important is the water content of the soil, 

 a feature which is of vital importance to the physiological processes 

 of the plant. 



The soil-water is not alone the only source of water for terrestrial 

 plants but is also the source of their mineral and nitrogenous food- 

 substances. The aerial pai'ts of plants are constantly losing water 

 by transpiration, a process analogous to evaporation but subject to 

 control by the plant within certain limits. Such water-loss is met by 

 the supplies of water derived by root-absorption from the soil-water. 

 In order that a plant may maintain its turgidity and perform all its 

 nutritive processes in a normal manner it is necessary that the 

 amounts of water absorbed and transpired be nearly equal. The 

 amount of transpiration exhibited by a plant is partly due to its 

 specific characteristics of leaf size, leaf structure, etc., partly to 

 various physiological controls exerted by the plant to a different de- 

 gree at different times, and partly to the physical factors of the en- 

 vironment, heat, humidity, light (as transformed to heat in the 

 leaves) and wind. These atmospheric factors are much more uni- 

 form over any county of Maryland than are the amounts of soil- 

 water. The fact that all plants actually do maintain a balance 

 he1 ween absorption and transpiration in the face of the uniformity of 



