28] 



CHAPTER II. THE TISSUES. 



95 



the transverse wall is oblique, as generally in woody plants 

 (Fig. 75). Each sieve-plate is a thin area of the wall which is 

 perforated by a number of closely placed open pits. The sieve- 

 plate is covered on both surfaces and the pits are lined by a 

 peculiar substance termed callus (Figs. 74 (7 c; 75 BCc), which 

 at least in. many plants periodically closes the pits in autumn. 

 Sieve-plates may also occur on the lateral walls. The rest of the 

 wall of the sieve-tube is rather thin : it is never lignified, but 

 consists of cellulose. The long straight sieve-tubes are connected 

 in their course by short transverse branches, so that they form one 

 continuous system of tubes. 



FIG. 74. Sieve-tissue of an herbaceous Angiosperm (Cucurbtta Pepo). A Transverse 

 sieve-plate in surface- view; B in longitudinal section; C sieve-plate closed by a plate of 

 callus c; c* sieve-plate on lateral wall, closed by callus; D contents of tube left after 

 solution of the wall by sulphuric acid ; s companion-cells ; pr lining layer of protoplasm ; 

 u mucilaginous contents. (x540: alter Strasbnrger.) 



In their normal active condition each segment of the sieve-tube 

 is lined by a layer of protoplasm (Fig. 74 B pr), in which starch- 

 granules are sometimes to te found, enclosing some mucilaginous 

 substance ; there is, however, no nucleus present ; the reaction of 

 the contents is alkaline. 



With the sieve-tubes of Angiosperms are closely associated cells, 

 termed companion-cells (Fig. 74 s), which are filled_with granular 

 proteid contents and have well-marked nuclei; each companion- 



