752 Marvels of the Untverse 
visible. The formation of the different types of clouds, each at about an average general level, 
however, indicates the existence of some of the great air-currents whose extent and speed have 
been more definitely shown by kite observations. A thorough knowledge of these is evidently a 
desideratum in these days of aerial navigation, since they will prove of far greater importance to 
the aeronaut than the winds and ocean currents are to the sailor. 
VEGETABLE HEDGEHOGS 
BY EDWARD STEP, F.L.S. 
PLANTs have such a marvellous power of adapting themselves to their environment that, however 
adverse the conditions may be in any particular spot, you are almost certain to find that some form 
or other of plant-life has discovered 
that it is unoccupied territory and has 
determined to annex it. There it 
is flourishing in its own way. The 
adaptations necessary to existence there 
have brought out such well-marked 
characters that if you show an un- 
known plant to an experienced botanist, 
he will be able to tell you within a little 
under what conditions it was growing. 
The varnishing of a leaf, the turning 
of its surface into velvet or clothing 
it more sparsely with long hairs; the 
spreading of its substance out broad 
and thin, or contracting it into a 
round, fleshy lump—all these varying 
forms and conditions have direct rela- 
tion to the circumstances surrounding 
the plant’s growth. 
As examples of one particular form 
of adaptive change, let us take the 
family of plants of which a few species 
are shown in the accompanying photo- 
— = graphs. They are a very numerous 
ieee A CACTUS IN FLOWER. Lesa family, known under the general name 
Many of this family bear magnificent wax-like blooms often of brilliant of Cacti; but the botanist has had to 
colouring. In this case, however, the flower is pure white. divide them up Tatomoriallern groups or 
genera, each with a distinctive name, such as Cereus, Opuntia, etc. The Cacti are all inhabitants 
of hot, arid places, where rain falls on very few days in the year, and where animal life is scarce 
to maintain it. It follows that 
because there is not enough vegetation—the basis of all animal life 
the few plants that have by their ingenuity contrived to exist under such forbidding conditions are 
much sought after as food, so that they have had to meet a two-fold difficulty—how to conserve 
sufficient moisture in their tissues to last them from one rainy season to the next, and how to ward 
off the attacks of animals. 
An ordinary plant growing under average conditions, where its roots can suck up as much moisture 
is constantly giving off large quantities of water in 
as it requires—usually more than it requires 
the form of vapour from innumerable minute pores in its leaves. If the water-supply at the root 
falls off or fails, the transpiration from the leaves still goes on, with the result that they and all 
