DIV. I 



MORPHOLOGY 



tinuous conducting channels. Such tissues are always associated in a 

 connected system traversing the whole plant. 



(a) Sieve-Tubes. The elements composing the SIEVE-TUBES ( 45 ) 

 are arranged in longitudinal rows and connected by open pores which 

 appear to serve for the transport of proteids and carbohydrates. The 

 transverse or oblique ends, and sometimes the lateral walls, have sieve- 

 like perforated regions the pores of which are filled with thick 

 protoplasmic strands. These are called the SIEVE-PLATES (Fig. 65 

 A, B). In many plants (e.g. the Cucumber, Fig. 65 A) the entire 

 transverse wall forms one area perforated by relatively coarse pores. 



, 



FIG. 65. Parts of sieve-tubes of Cucurbita Pepo, hardened in alcohol. A, Surface view of a sieve- 

 plate. B, C, Longitudinal sections, showing segments of sieve-tubes. D, Contents of two sieve- 

 tube segments, after treatment with sulphuric acid, s, Companion cells ; u, mucilaginous 

 contents ; pr, peripheral cytoplasm ; c, callus plate ; c*, small, lateral sieve-plate with callus, 

 (x 540. After STRASBURGER.) 



On the longitudinal walls the sieve-plates have the form of narrowly 

 circumscribed circular areas with much finer pores (Fig. 65 (7, c*) where 

 two sieve -tubes adjoin laterally. In other cases several finely- 

 perforated areas (sieve-plates or SIEVE-PITS) are found on the oblique 

 end wall of a sieve-tube (Fig. 66). The elements of a sieve-tube 

 (Fig. 65), each of which corresponds to a cell, contain a thin living 

 protoplasmic layer lining the wall, with a single nucleus, leuco- 

 plasts, and often starch grains. This encloses a watery, alkaline, 

 more or less concentrated, and coagulable cell sap which is rich in 

 albuminous substances and frequently in carbohydrates and inorganic 

 salts (phosphates). The walls of sieve -tubes are almost always 

 unlignified ; they .consist of cellulose and are elastically stretched by 



