i So 



BIOLOGICAL LECTURES. 



plant can get but little moisture, it also retains that little, or 

 lets it go with great difficulty. Hence, we have on the sea- 

 shore a vegetation corresponding in some respects to that in 

 high altitudes, or to that in desert regions, although the 

 apparent conditions are radically different from either. 



Hydrocotyle umbellata is a plant common in moist or watery 

 places from Maine to Florida. In the semi-tropical fresh-water 

 marshes of Florida, it places its slightly thickened, rounded 

 leaves at right angles to the sun's rays, parallel to the surface 

 of the water in which it grows. When it happens to encroach 

 upon the salt-water marshes of this same region the difficulty 

 of water supply is so heightened that the slender petioles 

 all make a right-angled turn at their upper end and put 



FIG. a. 



FIG. b. 



PLATE No. 14. 



the now thickened leaves in a vertical position, in order to 

 avoid the direct rays of the sun, thus lessening the loss of 

 water. 



Plate No. 14. fig. #, shows a sketch of this little plant, taken 

 from a fresh-water stream in South Florida. Fig. b on the 

 same plate shows another plant taken from a salt marsh near 

 by. All of its leaves are rendered vertical by this turn in their 

 petioles. The microscopic differences are much greater than 

 this relative difference in position. The leaves of the one in 

 the salt marsh have become much thicker, the epidermal tissues 

 much heavier, the number of the palisade cells has increased, 

 the intercellular spaces have nearly disappeared, and the 



