46 
PLANT MORPHOLOGY. 
are composed of cellulose. It consists of superimposed 
elongated cells, the transverse walls of which possess 
numerous pores which are supposed to be in the nature 
of openings, permitting of the direct passage of the 
contents from one cell to the other. This transverse 
wall, which may be either horizontal or oblique, is 
known as the sieve plate, and the thin places, as pores 
of the sieve. The sieve plates are sometimes also 
formed in the longitudinal walls. On the sieve plates 
of plants, the activities of which are suspended during 
the winter, there is formed on either side of the plate a 
layer of a colorless, mucilaginous substance, known 
as callus, which has somewhat the appearance of col- 
lenchyma, but is colored brownish by chlor- zinc- 
iodide. 
The sieve cells contain an albuminous substance 
somewhat resembling protoplasm ; in some instances 
starch grains have also been found. 
When the activities of the sieve tubes have ceased, 
they become altered in shape, owing to the activities 
connected with growth in neighboring cells, as well as 
in other parts of the plant, and are then known as 
obliterated sieve. In the drying of plants a similar 
alteration is produced, and for this reason the sieve of 
vegetable drugs is of this character. 
IV. PROTECTING CELLS. 
Protecting cells include those cells which are located 
on the outer parts of the plant. The function of these 
cells is to lessen the rate of transpiration, or the giving 
off of water ; to furnish protection against changes of 
temperature, and to protect the inner tissues against 
the attack of insects ; they also have a mechanical 
function. 
Depending principally upon their composition, these 
