90 the plant life of maryland 



General Considerations. 



The floristic distinctness of the three Zones of Maryland rests on 

 the fact that their boundaries are the collective limits of geographical 

 distribution of considerable numbers of species. Not only are cer- 

 tain species limited absolutely in their range by the "Fall-line" or 

 by the contour of 1500 feet elevation, which are the natural lines 

 bounding the Zones, but others are abundant on one side of these 

 bounds and rare on the other: in other words some plants terminate 

 abruptly in their range, while others merge from abundance through 

 in frequency to rarity, and at length disappear. Liqiiidambar 

 styraciflua, CleUira alnifolia and the other plants listed on page 76 

 are examples of such as terminate abruptly in range, while Tsuga 

 canadensis merges from abundance in the Mountain Zone and the 

 Upper Midland District through scarcity in the Lower [Midland 

 District to rarity in the Coastal Zone. 



The importance of temperature in determining the bounds of plant 

 distribution has long been recognized and is still being overempha- 

 sized by certain writers. The importance of conditions directly or 

 indirectly determined by the character of the soil in controlling the 

 local distribution and occurrence of plants has also been appreciated 

 since the writing by DeCandolle of his "Essai ftlementaire de 

 Geographic Botanique" in 1820. While the immediate influence of 

 climatic factors, particularly temperature, in determining the neu- 

 graphical distribution of plants is by no means to be underestimated, 

 it. needs, nevertheless, to be borne in mind that local soil conditions 

 not only determine the occurrence of plant species within their area 

 of distribution, but in many cases play a part also in determining the 

 exact position of their limit of geographical distribution. An 

 inquiry into the nature of the habitat of several species which have 

 the same geographical range will again and again show the habitat. 

 of these species to be the same, as is true of the pine barren plants of 

 the southeastern states and the bog plants of the northern states, the 

 geographical distribution of these species being largely dependent on 

 the distribution of the particular physical conditions which are favor- 

 able to their development. 



