176 



SIEVE-TUBES. 



by ihe above-mentioned secondary bast of ligneous Dicotyledons, e. g. Phytocrene, 

 Bignonia, Tilia, Juglans, Yitis (Figs. 69-70), Betula, Populus, Pirus communis \ &c., 

 the very oblique terminal faces of Calamus (Fig. 71), Philodendron Imbe, &c. In the 

 secondary bast of Vitis the occasional horizontal ends of members have also a 

 ladder-like structure. 



Sieve-plates are distributed in different ways, according to special cases, on the 

 lateral fiices of members of tubes, where these adjoin other similar members. 



On most forms with simple transverse plates, as Cucurbita, they are not un- 

 commonly absent on the sides, or they occur irregularly, and are then usually 



FIG. 69. FIG. 70. FiG. 71. 



Figs. 69. 70. — Vitis vinifera, Bast of a branch several years old, !<=" in thickness, in Summer (begfinniiig of July). Fig. 69 

 (145) Tangential section, s, s sieve-tubes, the inclined terminal surfaces, and a horizontal scalariform one, are cut through longi- 

 tudinally, with the exception of one at the upper edge, which is seen obliquely in superficial view, tn, ni medullary rays. Fig. 70 

 Radial section, two scalariform terminal surfaces of sieve-tubes in superficial view, separated from one another by narrow paren- 

 chymatous cells (375). 



Fig. 71.— Calamus Rotang (Spanish Reed). Fnd of a member of a sieve-tube isolated by maceration (375). 



relatively small. Where the lateral wall adjoins elements of another category iso- 

 lated, usually flat, pits are found : in these forms especially the lateral wall is very soft 

 and extensible ; after maceration in potash its inner layer may be drawn out to some 



' Compare Voii Mohl, I.e.; Dippel, Mikroskop, p. 251, &c. — Von Mohl's fig. 11 of Pirus repre- 

 sents the partial surface-view of three oblique terminal faces. 



