STEMS 



68i 



Figs. 1004-1008. — Vascular elements of an angiosperm: 1004, spiral vessels; 1005, 

 spiral and annular vessels ; ioo6', a pitted vessel ; 1007, a sieve tube with companion cells ; 

 looS, a cross section of a sieve tube at the sieve plate, showing also a companion cell in 

 cross section; 1004-1007 are from longitudinal sections ; all figures highly magnified. — 

 1004 and 1005 after Bonnier and Leclerc du Sablon; 1006 after DeBary; 1007 and 

 1008 after Strasbdrger. 



a thin margin, the margo, the latter representing the space between the rings as 

 seen in surface view. When the thickening of tracheids and tracheae is completed, 

 the cell contents die, after which the lumen contains water and solutes, together 

 with numerous air bubbles. In old wood the lumina contain largely air, although 

 various excreted products often 

 accumulate. 



Sieve tubes and conductive 

 parenchyma. — Siei'e tubes are 

 syncytes, occurring in con- 

 tinuous rows, like tracheae, 

 but differing therefrom in 

 their thin cellulose walls and 

 in their living and highly 

 albuminous viscous contents 

 (fig. 1007). They rarely ex- 

 ceed two millimeters in length, 

 in this respect resembling tra- 

 cheids rather than tracheae. 

 A unique feature of sieve 



Fig. 1009. — Cross sections of sieve tubes of 

 a gourd (Lagenaria vulgaris), showing sieve 

 plates with large pores (a) and with small pores 

 (6); highly magnified. — From DeBary. 



