136 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[ 29. 



the contiguous segments communicating by means of the sieve- 

 plates, which usually lie on the transverse walls, either singly, 

 when the transverse wall is horizontal, as generally in herbaceous 

 plants (Figs. 97), or several together, when the transverse wall is 

 oblique, as generally in woody plants (Fig. 98). 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 sur- 

 faces, and the pits are lined, by a peculiar substance termed callus 

 (Figs. 97 c; 98 B C c; 99), which at least in many plants peri- 

 odically closes the pits in autumn. Sieve-plates may also occur 

 on the lateral walls (Fig. 99). The rest of the wall of the sieve- 



B 



Fie. 97. Sieve-tissue of an herbaceous Angiosperm (Cueurbifa Pejio). A Transverse 

 sieve-plate in surf ace- 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 ; 

 gelatinous contents. ( x 540 : after Strasburger.) 



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 sys- 

 tem of tubes. 



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

 is lined by a layer of protoplasm (Fig. 97 B pr), in which starch- 

 granules are sometimes to be found, enclosing some gelatinous sub- 

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

 contents is alkaline. 



