786 FUNCTIONS OF EPIDERMAL TISSUE. 
ance of these vessels has given rise to much discussion, and is 
still involved in obscurity. But it would appear that these 
vessels, and others which are closely allied to them, as sieve- 
tubes and vesicular vessels, act as temporary reservoirs of nutrient 
fluids, and also as carriers of such fluids to those parts of plants 
where they are required. (See also page 51.) Schultz called 
the tissue formed by the ramifications of the laticiferous vessels 
cineinchyma, because he believed that he had discovered in it a 
peculiar vital movement or circulation of the latex, to which he 
gave the name of cyclosis.* Other observers have also described 
a similar circulation ; but Mohl, Henfrey, &c., altogether deny 
the existence of such a movement in uninjured tissues. 
Dr. A. Fischer states that it is only the young cells of sieve- 
tubes which can produce albuminous substances, since they 
possess nuclei which are absent in the mature tubes. 
4. Functions oF EpipERMAL Tissuzr.—The special functions 
of epidermal tissue are :—to protect the tissues beneath from 
injury, and from being too rapidly affected by atmospheric 
changes ; to regulate the transpiration or exhalation of watery 
vapours ; to absorb and exhale gaseous matters ; and probably, 
to some extent, to absorb water. The epidermis itself is specially 
designed to prevent a too ready evaporation of fluid matters 
from the tissues beneath, and hence we find that it is variously 
modified to suit the different conditions to which plants are 
submitted. ‘Thus, in submersed plants and submersed parts of 
plants, which are always exposed to similar influences as regards 
moisture, there is no true epidermis; whilst in aerial plants 
submitted to ordinary influences in cold and temperate climates, 
we generally find an epidermis with only one layer of thin-sided 
cells, and covered by a cuticle of only moderate thickness, 
Cellulose is rarely, and then only with difficulty, discovered in 
cuticle, which is a thin structureless membrane extending un- 
interruptedly over the boundaries of the subjacent epidermal 
cells. It is coloured yellow by Schulze’s fluid, yellow or yellow- 
brown on the addition of iodine, with or without sulphuric 
acid ; it is soluble in boiling caustic potash, but insoluble in 
concentrated sulphuric acid. In other aerial plants, however, 
growing in the same latitudes, such as the Box, &c., and 
generally also in those of a succulent nature where there is but 
a moderate exhalation, we find the upper walls of the epidermal 
cells especially thickened, or protected by a dense layer of 
cuticle ; whilst in aerial plants growing in very dry or hot 
regions, as the Oleander (fig. 125), we have frequently an epi- 
dermis of two, three, or more layers of thick-sided cells, and 
other special contrivances to prevent a too ready exhalation 
of fluid. For instance, De Bary states that wax may be de- 
posited in the cuticle, and that on heating to about 100° C., it 
* The term ‘cyclosis’ has also been applied to the movement of proto- 
plasm in cells, (See p. 784.) 
