NorrH Haven SAnp PLarins 613 
Desert plants (xerophytes) have many peculiar adaptations for 
the conservation of water. Large parenchyma-cells, or increased 
palisade tissue, become abundant, in many cases forming a fleshy 
or succulent plant which reaches the highest development in the 
Cactaceae. 
But many plants have developed special cells for water-holding 
purposes, while others through their root system have provided 
a ready means of taking up water and some have a combination 
of both arrangements. This is known as a xerophytic structure 
and is found in desert plants, though it is not uncommon to find a 
similar structure in beach plants or even hydrophytes. Sadicornia 
and certain species of Atriplex which have succulent stems or 
leaves will serve to illustrate this point. It has been explained 
that, as these plants commonly grow on salt marshes and are often 
covered by the tide, on account of the saltness of the water the 
plants may not be able to make use of it and really depend upon 
showers and the dew for their water supply ; hence the necessity 
of conserving this until more water may be obtained. 
It is also claimed that swamp plants develop similar structures 
on account of the accumulating quantity of acid derived from the 
humus of the soil through chemical changes in it; that even 
Swamp plants on account of this acid may not be able to obtain 
plenty of pure water and these structures are developed for the 
purpose of economy in using it. 
Be this as it may, the fact remains that we do find the same or 
similar structures in plants of quite different habitats. Cowles 
found that willows, which are perhaps the most typical of swamp 
trees, were best able to adapt themselves to the conditions of the 
advancing sand dunes of Lake Michigan * because when partially 
buried by the sand to the depth of many feet the tree put out 
new roots and was not injured. Many other trees like oaks and 
pines were killed. Schimper goes still farther in showing the simi- 
larity between xerophytes and bog plants, and also states that there 
are no structural differences by which we can distinguish xero- 
phytes from halophytes. Where differences exist they are chiefly 
Physiological in character and relate to the effect of light, heat and 
a moist or dry air upon the plant. ‘ Physiological dampness then 
* Cowles, H.C, Bot. Gaz. 27: 292. 1899. 
