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- FIG. 17. i 



erous tissue. 



tern 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 



