CHAPTER II. PLANT TISSUES. 



I6 7 



away from the sieve-plate. This matter is 'continuous from one 

 cell to the next through the perforations in the plates; thus 



sieve cells, during 

 the growing sea- 

 son, at least, form 

 long, continuous 

 tubes, through 

 which nutritive 

 materials circu- 

 l, late in the plant. 

 The sieve-plates 

 are usually con- 

 siderably thick- 

 ened, often with 

 a deposit of a 

 peculiar charac- 

 ter called callus, 

 but the rest of the 

 Fig. 429. cell-wall remains 



very thin, and is a 



composed of un- 

 modified cellulose. 



IV. -Laticifer- 

 ous Series. Many 

 plants, when wound- 

 ed, emit a milky 

 fluid, varying in col- 

 or,copiousness, con- 

 sistency and chemi- 

 cal composition in 

 different plants. The 

 tissue which con- 

 tains the fluid, or 

 latex, is called lati- 

 ciferous tissue. 



Fig. 430. 



Fig. 429.— Simple laticiferous tissue from the root of Apocynum androssemifolium. 

 a. milk tube containing coagulated latex; i, cortical parenchyma cells. Magnified 155 

 diameters. 



Fig. 430.- Complex laticiferous tissue from longitudinal tangential section of root of 

 Dandelion, a, milk-vessel; b, parenchyma cell. Magnified 155 diameters. 



