340 
Laticiferous tissue consists of branched tubes somewhat resembling the 
veins of animals. They occur in the inner bark, and contain a fluid called 
latex, which is often white, yellow, or reddish. Lindley says the use of this 
tissue is unknown. It is said to carry the latex to all the newly formed organs 
which are supposed to be nourished by it. 
Compound Organs.— The elementary organs, now briefly described with 
some modifications not requiring to be noticed here, are variously combined to 
form the compound organs. These may be considered as nutritive and repro¬ 
ductive. The nutritive consist of the stem, roots and leaves, which are occupied 
in the nourishment of the plant. The reproductive, the flower and fruit which 
produce new individuals. 
Flowering plants are covered externally by a thin membrane, the epidermis. 
It consists of one or more layers of cellular tissue and covers all parts exposed 
to the air, excepting the stigma of the pistil and the extremities of newly- 
formed roots. The epidermis is itself covered by a very thin membrane, the 
cuticle. Plants growing in a dry climate have generally a very thick epider¬ 
mis. It is comparatively thin on those plants which prefer damp, shady situa¬ 
tions, and is entirely absent from leaves and other parts which live constantly 
under water. Hence it has been concluded that the use of the epidermis and 
cuticle is to protect the tissue in the interior of plants from the immediate 
action of the air. 
The epidermis is pierced with small openings or mouths called stomata. 
They occur on the green parts of plants, especially leaves. They are said to 
open or close according to the hygrometric state of the atmosphere, and are 
considered to regulate evaporation and respiration, especially the former. The 
number on a square inch of leaf has been counted on many plants, and is found 
to vary very much in different species, and in different parts of the same plant. 
Stomata are most abundant generally on the under surface of leaves; many 
plants have no stomata on the upper surface. The grape-vine and the lilac for 
instance have none on the upper side, while the former has 13.600, and the 
latter 160,000 on the lower surface. When leaves grow vertically, the number 
of stomata is often equal on both sides.. The spider-wort of our woods (Tra- 
descantia) is said to have 200 on a square inch of the surface of each side of 
the leaf. Stomata are formed on the upper surface only of floating leaves, and 
they do not usually exist on leaves which are always submerged. 
