4 
POPULAR SCIENCE REVIEW. 
or cut into slender divisions. Maritime plants growing within 
reach of the salt spray are apt to become dwarf and fleshy in 
their habit; and the same remark applies to those which 
grow on exposed mountain summits, where they are liable to 
severe though short droughts during the brief but intense 
summer. In arid desert situations this feature of the vegeta- 
tion is still more remarkable. Our yellow and white stone-crops, 
with their round juicy leaves, lovers of rocks and dry walls, 
are replaced, as we go farther south, by larger species of the 
same order, or by the similarly disposed Ficoidese, as the pretty 
little Mesembryanthemum crystallinum , the ice-plant of our 
greenhouses, which refreshes with its cool foliage the borders 
of the desert in Egypt, and elsewhere in North Africa. Many 
orders of plants, indeed, occurring with us only as ordinary 
herbs or slender shrubs, are represented in those countries by 
genera of succulent plants, great favourites in our greenhouses, 
whose affinity it is hard to recognise. 
One of the most remarkable features of the hotter and drier 
parts of America is the abundance of different forms of Cactus, 
so much cultivated in this country for the beauty of their 
flowers and the singular weird form of their trunks, which 
perform the functions of both stem and leaves. Having its 
head-quarters in Mexico, the order extends as far as the tempe- 
rate latitudes of Chile and Canada, and includes, on a moderate 
computation, at least one thousand species. In Africa the 
order is entirely absent, or rather its absence is made more 
conspicuous by the occurrence of a single species of Rhipsalis 
at the Cape ; but its place is supplied by another class of plants, 
the Euphorbias , a genus represented in this country by seve- 
ral inconspicuous but familiar weeds known as Spurges. In 
tropical and subtropical Africa the genus assumes the habit 
and general appearance of the absent Cacti , though in their 
botanical affinities they are nearly as remote as two orders of 
plants can well be. Except when they are in flower, it is, 
indeed, difficult to believe that these African Euphorbias are 
not in reality Cacti ; and the resemblance is not merely a 
general one ; particular groups, and even species, of African 
Euphorbia imitate particular groups or species of American 
Cactus in the form and habit of the stem and the arrangement 
of the spines, so that it is almost impossible to distinguish 
between them. This singular imitation is not, moreover, 
confined to these two families. The accompanying illustration 
(Fig. 1), reminding one irresistibly of a familiar Cactus , is drawn 
from a species of Stapelia , allied to S. hirsuta , belonging to 
the order Asclepiadacese, a near ally of the brilliant and fragrant 
Stephanotis and Hoy a of our stoves, and equally remote, in any 
system of classification, alike from the Cactacese and the- 
