PLANT TISSUES 141 
Stems and leaves of Dicotyledons and roots of Dicotyls and 
Gymnosperms of secondary growth. In stems and roots the 
phloem faces toward the exterior and the xylem toward the 
center of the axis. 
IV. Bicollateral, characterized by a xylem strand befng 
between an inner and an outer phloem mass. Cambium may 
; VG : 
Fic. 79.—Closed collateral bundle of stem. of Zea mays. VG, Bundle sheath; 
L, intercellular space; A, ring from an annular tracheal tube; SP, spiral tracheal 
tube; M, pitted vessels; V, sieve tubes; S, companion cells; CP, crushed primary 
sieve tubes; F, thin-walled parenchyma of the ground or fundamental tissue. 
(From Sayre after Strasburger.) 
be present or absent on either outer or inner face of the xylem 
or on both faces. Seen chiefly in stems and leaves of the Cucurbz- 
tacee, Combretacee, Loganiacea, Apocynacee, Asclepiadacee, Oleacea, 
Convolvulacee, Gentianacee, Myrtaceae and Solanaceae. 
V. Concentric, characterized by a central xylem mass sur- 
rounded by a phloem mass or vice versa. No cambium present. 
(a) Concentric, with xylem central in bundle. (Xylocentric 
bundle.) Seen in stems and leaves of nearly all ferns and the 
Lycopodiacea. 
(b) Concentric, with phloem central in bundle. (Phlocentric 
bnudle.) Seen in stems and leaves of some Monocotyledons. Exam- 
ples: Calamus and Convallaria rhizomes. 
