296 A TEXT-BOOK OF BOTANY. 



rise to the deep furrows (Fig. 160) which are so characteristic 

 of the outer surface of our large trees. 



Laticiferous or milk tissue occurs in all those plants which 

 emit a milk- juice on being cut or otherwise wounded. The juice 

 may be colorless, as in the oleander; whitish, as in the Asclepia- 

 daceae, Apocynaceae, etc. ; or yellowish, as in the Papaveracese. 

 It contains caoutchouc, oils, resins, mucilage, starch, calcium 

 oxalate and alkaloids as well. The walls are relatively thin and 

 consist chiefly of cellulose. The tissue consists either of single 

 cells of definite length, as in the Papaveracese, or the cells may be 

 of indefinite length, as in the Asclepiadacese, or it may consist of 

 a more or less branching network (Fig. 127) formed by the 

 anastomosing of a number of cells, as in Taraxacum (consult 

 paragraph on Latex, pp. 238-241). 



As has already been stated, the latex of plants contains a num- 

 ber of plastic or trophic substances, i.e., those which, either at 

 once or after being stored for a time as reserve food, are drawn 

 into metabolism and serve as nutrient material. They also con- 

 tain a number of aplastic or non-trophic substances, as caoutchouc, 

 resin, alkaloids, volatile oils and tannin, which are in the nature 

 of metabolic by-products and are incapable of further metabolism. 

 While it is highly probable that the laticiferous tissue, on account 

 of its being always associated with the phloem, functions to some 

 extent for the transportation o>f plastic substances, yet it serves 

 another purpose, viz., to protect the underlying cells after injury 

 of the plant by insects or'herbivorous animals. This protection 

 results from the rapid coagulation of the exuding latex upon 

 exposure to the air and forming a varnish-like surface. In some 

 cases the latex contains a poisonous principle which exercises a 

 protective function. In Rhus Toxicodendron the principle causing 

 the eczema, namely toxicodendrol, is supposed to be formed in 

 laticiferous tissues being transferred to the hairs, which upon 

 being broken liberate the poison. 



