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DISTRIBUTION OF AQUATIC PLANTS. 17 



best zonal plants, and afterwards to arrange the other loss important 

 ones in their proper places in the schedule. 



The first of the two characteristics of a good zonal plant is that it 

 shall have a definite termination at the borders of a zone, or at lines sub- 

 stantially parallel thereto but closer together. For example, Larrea 

 tridentata occurs throughout all except the highest altitudes of the 

 Lower Sonoran zone, while Grayia spinosa occupies only a compara- 

 tively narrow but well-defined strip, or belt, in the upper portion of this 

 zone. The latter species, although confined within closer limits, is 

 nevertheless an important zonal plant, for it indicates, if not the limits 

 of the zone, certain definite points within them. The area actually oc- 

 cupied by this plant may be called the Grayia belt. 



Secondly, the belt of a zonal plant should be continuous. It frequently 

 happens that an area may be perfectly adapted to the support of a 

 certain species but that other plants of more robust growth have 

 crowded it out. When such a condition of affairs exists over large 

 areas, the zonal relations of the species may be obscured and the plant 

 be of little value in this respect. Furthermore, a zonal plant should be 

 able to grow on as great a variety of soils as possible, for one portion 

 of a zone is likely to differ in this respect from another. It is a matter 

 of common experience that the vegetation of a sandy soil is strikingly 

 different from that of an adjacent clayey soil. 



If the plants of any region be tested by these two principles, most 

 of the nonvaluable species may be eliminated. 



Aquatic plants in most cases possess neither of the characteristics 

 described above. Their place of growth, in streams or other bodies of 

 water, necessarily confines them to small portions of a region's surface; 

 while a far more important fact — that the controling element in their 

 distribution is the temperature of water, not the temperature of air — 

 renders possible their growth at almost any altitude or any latitude. 

 It is worthy of remark that the distribution of aquatic plants may be 

 expected to accord approximately with that of fishes, in a manner an- 

 alogous to the known general accordance of the distribution of terres- 

 trial plants with that of terrestrial animals; for one factor of the high- 

 est importance, temperature of air, is common to the two groups of 

 terrestrial organisms, while the temperature of water is the main con- 

 trolling influence with the two aquatic groups. It was found to be true 

 that not only the aquatic species but also the marsh plants of the des- 

 ert are almost without exception those of similar habitat in other less 

 arid regions. In vertical distribution, too, a similar lack of confinement 

 within floral boundaries is constantly observed. On the banks of moun- 

 tain streams occur many species not confined to one zone, but extend- 

 ing over two or three. 



The group of plants shown by experience to be most reliable as zonal 

 guides is composed of the trees and shrubs. A little thought directed 

 to the conditions which contribute to the growth of such plants explains 

 13095— No. 1- 1! 



