Lithops - Au Cactus Francophone

structure referred to above, is more or less circular or semilunar in shape and
its surface varies from apparently opaque to transparent.
Walter (4) determined the amount of moisture taken up and lost by
Lithops salicola. He found that in one case a large plant (weighing 11.50
gm.) lost per diem when fully exposed to the sun scarcely 0.1 gm. in weight.
Another plant (weighing 4.395 gm.) lost in 15 days scarcely 0.2 gm. Walter
also determined the amount of water taken up by the roots of these plants.
By placing a plant so that the roots were immersed in water, he found that
a plant weighing 23.95 gm. had taken up 3.3 gm. of water in 15 days. By
making a small cut at the base of the plant and putting it in water, this plant
had absorbed 7 gm, of water in 24 hours. It thus appears that the whole
surface of the plant, including the root system, is practically impervious to
water. Walter also found that a plant gained scarcely 0.1 gm. per day when
fully submerged in water. Absorption of water takes place apparently through
the newly developed roots and not through the old roots.
This protection against the loss of water is of very great advantage,
because most of the species of this genus grow in the most arid parts of
the country, where the relative humidity of the atmosphere is very low, and
consequently plants would lose all their moisture if not protected against such
loss. Apart from the low humidity content, these areas very frequently have
rainless periods of 12–20 months or even longer.
Green plants, as is well known, are dependent on light for the manu
facture of their food. In this genus the plant is buried in the soil and if the
exposed top surface were green the light could not penetrate very deeply into
the tissues and thus the greening of the underlying cells would be prevented.
If the chlorophyll were confined to the top surface of the leaf this would be
the same as a reduction in leaf surface, and consequently the amount of solar
energy available for the manufacture of the food would be very small in
deed. This difficulty could be overcome by increasing the top surface, but that
would be attended with the additional dangers of desiccation and heating of
the body of the plant, especially as these plants are embedded in a soil which
is also exposed to the full rays of the sun. In species of Lithops an interesting
way out of this difficulty has been found. The chlorophyll containing cells line
the inner surface of the mantle and thus lie for the most part below the soil
level, and all the light which reaches them must come from above through
the window like top (Fig. 3). In some species the top surface is transparent
(L. optica, L. olivacea, L. marmorata), in others there are a number of mini
ature windows (L. fulviceps), i.e. small transparent areas with opaque tissue
between, while in other species (L. Meyeri) the top surface, though opaque,
yet allows sufficient light to pass through. Schanderl (2) has found that in the
case of Fenestraria rhopalophylla, where the window resembles that of L. optica
and L. olivacea, 90–91.5 per cent. of the incident light passes through, and at
a depth of 17, mm. only diffuse light is found. In other words the transparent
window lets through a very considerable quantity of light.
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