220 REV. @ HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
when seeds of desert plants are sown in ordinary garden-soil 
many fail to grow at all*; just as, while some water-plants 
grow more vigorously on land, others cannot live if exposed to 
the air. Or, again, with regard to many maritime plants, they 
will grow quite as well, if not better, away from the sea, either by 
altering their structure as Samphire does, by developing flat and 
thin leaves, or else they may retain their usual features, this being 
due to heredity. Thus Salsola Kali, the Prickly Glasswort of 
our sandy sea-shores, has become one of the worst weeds ever 
introduced into American wheat-fields. One year’s damage in 
Dakota alone is estimated at 2,000,000 dollars. Itis described as 
taking complete possession of the soil, while its spiny nature 
makes it objectionable to horses and other animals (/armer’s 
Bulletin, No. 10, 1893). In this case the plant has not lost its 
spines by growing in a richer soil, and illustrates the fact that 
hereditary influences often, if not always, tend, more or less, to 
resist the effects of a changed environment. And just as new 
adaptations can easily become fixed in some cases, but with great 
difficulty in others, as in cultivating wild plants ; so, conversely, 
while some features are instantly lost, others are as rigidly 
retained, though it may be in a modified form. What, however, 
may be called the general “ plasticity ” of plants is now so well 
recognized, that it affords us a perfectly adequate means in 
accounting for the self-adaptation of plants, although it is far 
from being necessarily applicable in every feature. 
I will now give some special characteristic features of desert 
plants, and then compare them with others growing in arid 
districts to show the more or less great similarity which 
prevails. 
II. General Morphological Characters‘. 
On first entering the desert near Cairo, where plants are to 
be found,—namely, along the lines of watercourses, which are 
* According to Dr. E. Sickenberger in Cairo and to my own experiments at 
home. 
t The best work Iam acquainted with on the structure of desert plants is 
one by Dr. G. Volkens, entitled “Die Flora der aegyptisch-arabischen Wiiste 
auf Grundlage anatomisch-physiologischer Forschungen dargestellt,” and the 
epitome “ Zur Flora der aegyptisch-arabischen Wiiste, eine vorliufige Skizze.” 
He gives eighteen plates (4to) illustrative of the anatomy of a well-selected 
series of types. I have fortunately been able to collect a large number of 
cne same piants myself n the deserts around Cairo; and for nearly all the 
