HEW PHYTOIiOGIST. 
Vol. VIII., No. 2. 
February 27TH, 1909. 
HYDRODYNAMIC FACTORS INFLUENCING PLANT- 
LIFE ON SANDY SEA SHORES. 
By Pehr Olsson-Seffer, Ph.D., 
Mexico City. 
N a recent paper 1 I have used the expression hydrodynamic 
factor to signify the influence of water in the soil upon plant- 
life as distinct from edaphic and atmospheric influences. The 
water-content of the substratum is recognized as one of the most 
potent factors in plant-life, and in the study of vegetation on sand- 
formations and the conditions there existing, the peculiar relations 
of sand to the water contained in the soil will at once force them¬ 
selves upon the observer. 
Sand, of all natural uncultivated soils, ranks highest in porosity, 
and lowest in capacity of retaining water, and, as is well known, it is 
the retentive power of a soil which determines its suitability to a 
certain kind of vegetation. It is the volume of water a soil is 
capable of placing at the disposal of the plants, which is the 
limiting factor in production of its vegetative covering and the 
controlling condition in the distribution of this vegetation. And it 
is not only as a factor in the growth of plants that water-content of 
the soil is important. In the case of sand, which under certain 
conditions is liable to drift, the soil-moisture is of the greatest 
consequence in checking this movement. Taking perfectly dry, 
pure quartz sand as zero, a mixture of only 4% of water gives it so 
much coherence that it may be pressed to a ball in the hand, and 
cease to be driven by the wind. 
Percolation of Water. 
But on account of its peculiar properties, sand allows the 
precipitation to percolate rapidly, and the acting force, gravi- 
1 Pehr Olsson-Seffer. “ Relation of soil and vegetation on sandy 
sea shores,” Botanical Gazette, Chicago (in the press). 
