BY SELF-ADAPTATION TO THE ENVIRONMENT. 225 
Phragmites communis. This grass is very abundant in the 
Nile Valley, growing in places which are not artificially 
irrigated. It covers large areas of waste ground outside Cairo, 
furming a stunted growth, the leaves being very short and sharp- 
pointed. It has been named var. isarica. Close to the Nile, 
however, in Rhoda Island, it grows ten or more feet high, with 
Jong leaves almost exactly like the plants in English rivers. 
In many places the two forms of leaf are on the same stem, 
sometimes even alternating with each other, suggesting the idea 
that the leaves were elongated or abbreviated and spinescent, 
according as the plant happened to have sufficient water at its 
disposal or not. ‘ 
In this plant we have, therefore, a varietal character which is 
quite inconstant, as it varies repeatedly even on the same stem, 
and which has not become relatively fixed, though deemed worthy 
of a name. 
Such and other facts show how completely relative many 
varietal and even specific characters may be. They may have 
every degree of constancy, but they can often and easily change if 
the environment be altered, till other characters take their place. 
which may then become relatively fixed in their turn. 
I have said that all observations tend to prove that the great 
reduction of the parenchymatous tissues, with a correlated 
hardening of the mechanical tissues—so that a spinescent cha- 
racter becomes very characteristic of desert plants—are simply 
the inevitable results of the action of the environment. That 
this is so, is corroborated by an experiment of M. Duchartre on 
the effect of drought upon Dioscorea Batatas *. . 
Some tubers of this plant produced long shoots, but without 
being allowed to have any water at all. The effect upon the 
stem was that it acquired excessive rigidity with no heliotropism 
nor power of climbing. Although the stem was much more 
slender than normally, there was a marked predominance of the 
elements of consolidation. The fibres had very thjck walls and 
a minute lumen. The periphery of the central cylinder showed 
a zone of perceptibly greater thickness than usual, consisting of 
fibres of small diameter, but with walls of greater thickness than 
usual. This appears to have been the usual pericycular zone so 
common in herbaceous endogenous stems. Similarly, the collen- 
* Bull. Soc. Bot. de Fr. 1885, p. 156. 
