LACTICIFEROUS TISSUE 39 
The central vacuole is filled with a liquid very rich in pro- 
tein matter, the masses of this protein substance often 
being continuous through the pores of the sieve plates 
with those of the adjoining sieve tubes. 
51. The sieve tubes of the Flowering Plants are 
accompanied by usually slender parenchyma cells full of 
protoplasm, the so-called companion cells. The walls 
between these and the sieve tubes are perforated by 
numerous very minute passages invisible except by special 
manipulation. Other forms of parenchyma cells are 
usually found adjacent to the sieve tissue. ‘The function 
of the sieve tissue is probably the transportation of 
protein substances from the leaves to parts of the plant 
where they are needed in the construction of new cells. 
Possibly, also, sugars are transported, at least in part, in 
the same tissues as well as in the ordinary parenchyma 
cells near them. The function of the companion cells 
is not certain. 
52. Laticiferous Tissue. This consists of a system 
of tubes extending throughout the plant 
and filled with a substance called latex. 
This is usually white (hence the name “ milk 
tissue”’ often applied to this kind of tissue), 
but may be colored red, yellow or even be 
almost clear and colorless. The latex con- 
sists of water containing usually much pro- Fie. 17.—Latieif- 
. erous tissue. 
tein matter as well as some sugar and 
salts dissolved in it, and holding in suspension numerous 
minute globules of resin or in many cases, caoutchouc. 
On exposure to the air, the latex often coagulates. It is 
from the latex of many plants that rubber and gutta 
percha are obtained, while other substances of great value 
are often found in it also, e.g. opium in the latex of 
the poppy. In some plants, starch grains are found in 
