98 
THE MEDITERRANEAN NATURALIST 
ly great length; so that while the upper layers of 
the sand may be heated to 140 degrees F., the plant 
can still flourish since the root fibres extend to 
great depths. Thus, a plant of Monsonia niveci of 
one year has a small rosette of three or four leaves, 
the roots of which may be 20 inches in length. 
The roots of some bushy plants have been found 
to be two yards and seven inches long. Some roots 
may even become twenty times the length of the 
stem. The Colocinth grows singly, has large soft ' 
leaves, without any means of preventing an excess i 
of transpiration, for a cut shoot fades within a few 
minutes; yet it flourishes unshaded through the 
whole summer. 
On some roots tuberous swellings occur, which I 
would seem to act as reservoirs of water; such 
occurs on species of Frodium , 
With regard to stems, they are often knotted and 
gnarled at the base with a stunted growth When 
they grow to good sized bushes, they are often 
spinescent. One of the largest, and a typical desert 
plant, is Zilla myagroides. In some cases they are 
almost or quite leafless, thus reducing the transpir- 
ing surface to a minimum. 
The anatomy of many stems shew the whole of 
the cortex to be converted into a colourless water- 
storing tissue; the elements consisting of long thin- 
walled cells, which store up water absorbed by the 
roots or leaves, or both, this cortex subsequently 
forming a dense layer of cork, Avhich acts as a pro- 
tection. In some cases, the pith acts as storage 
instead of the cortex. 
A common form of stem is the spine, but spiny 
processes are often homologous with leaves, as in 
Astragalus , Fagonia , &c. These spines, besides be- 
ing hard in consequence of the poverty of water, 
which always reduces the amount of cellular tissue, 
act as a storage of such water as the plants can get. 
The peculiarities of the leaves of desert plants 
are certainly the most noticeable, they being main - 
ly centred in securing protection against loss of 
water by transpiration. 
First as to the epidermis and cuticle. The latter 
is much thickened and often coated with wax, and 
strongly striated. The hairs are often entirely or 
partially coated with wax as well. 
Hairs form one of the most general kinds of pro- 
tection, often forming a dense felt over the surface 
when they are stellate, by their rays thickly inter- 
lacing. The hairs, besides acting as non-conductors, 
also act as absorbents as well, for dew is of course 
retained by a felt-like mass. Even when the hairs 
are coated with wax they may be partially free 
from it at the base, or gashed and riddled with 
holes through the waxy layer, allowing dew to be 
absorbed. The interior walls, both of the hairs and 
epidermal cells, may become more or less of a 
mucilaginous character by the absoption of water, 
and thus tend to retard subsequent evaporation. 
J ust as in all the storage tissues the water becomes 
thickened, so that its loss by evaporation is greatly 
hindered. In some cases there are jointed hairs 
which act as storage, e. g ., Atriplex desn'ti. If the 
water becomes exhausted, these hairs collapse and 
stick together, forming a parchment-like layer over 
the epidermis, affording a strong and excellent 
protection against excessive transpiration. 
The epidermis is often a storage tissue, the cells 
bulging both outwards and inwards forming 
bladder-like structures, scattered thinly or thickly, 
or in rows over the surface of the leaf. The ice-plant, 
which grows about Alexandria, is a familiar exam- 
ple. 
With regard to the form of the leaves, the size is 
mostly very small or even minute. If they be 
deeply divided, the lobes will be very narrow. The 
edges are often wavy or in rolled. When first 
formed on the commencement of the rainy season 
they grow larger, but during the drought these 
sometimes perish, and smaller leaves only are 
produced. 
The anatomical structure is also correlated to the 
environment. The chlorophyllous tissue is very 
dense, palisade cells being on both sides, while the 
mesophyl is compacted with polygonal green cells. 
In some case the mesophyl acts as a storage tissue. 
This occurs, e.g., in aloes. Under the epidermis is 
a large layer of chlorophyllous tissue, which covers 
the central mass of thin-walled rounded cells, 
containing a colourless semi-fluid matter, which, 
when, extracted, hardens into the bitter aloes of 
pharmacy; wdiile living, however, it appears to acts 
as a storage of water, the bitter substance held 
in solution probably preventing the water, 
from evaporating, just as in other plants it 
becomes mucilaginous or gummy. 
Besides the above features, one of the most 
remarkable is the secretion of salts, which are 
