134 



BOTANY 



PART I 



filled with water. They are internal hydathodes (cf. p. 119) and serve for the active 

 excretion of water. Water pores (p. 115) are usually situated above such epithemata. 

 The leaf-tips of a number of aquatic Monocotyledons show a depression into which 

 the terminations of tracheides project. These depressions arise by the destruction 

 of water-pores or of these together with the epidermis. They may be closed by the 

 persisting cuticle. These apical openings serve in the same way for the excretion 

 of water ('^). 



The Course of the Vascular Bundles (^^^). — The bundles exhibit 

 a definite course and arrangement within the body of a phxnt. It is 

 sometimes possible, by maceration, to obtain preparations in which the 

 course taken by the bundles may be followed. By allowing a leaf, 

 stem, or fruit to lie in water until it has become softened and 

 disintegrated, a skeleton formed by the more imperishable vascular 



system may be obtained. 



Vascular bundles which jmss from a leaf 

 into a stem form within the latter what are 

 known as leaf-traces. The leaf-traces may 

 be composed of one or more vascular bundles, 

 and are accordingly distinguished as one- 

 strand or many-strand leaf-traces. When, as 

 is usually the case, the vascular system of the 

 stem is entirely composed of leaf-traces, each 

 vascular bundle of the trace after passing 

 downwards for some distance unites with 

 another entering from a lower leaf. The 

 arrangement of the bundles in a stem varies 

 according to the distance and direction tra- 

 versed before the coalescence of the bundles 

 takes place. A relatively simple case is 

 afforded by the young twigs of the Dwarf 

 Juniper (Junipcrus nana) (Fig. 143). The 

 leaves are in whorls of three, the leaves of successive whorls alter- 

 nating with one another. From each leaf a leaf-trace consisting of a 

 single vascular bundle enters the stem. This divides into two about 

 the middle of the internode below, and the divisions diverge and 

 unite with the leaf-traces of the whorl below. The arrangement of 

 the bundles may be shown diagrammatically by representing the 

 bundles as if on the surface of an unrolled cylinder, so that they all 

 appear in one plane. This is done in Fig. 143, which also shows the 

 insertion of the vascular bundles of the axillary shoots (/.•) on those of 

 the parent shoot. 



The arrangement of the bundles in the Yew {'Ta.nis haccafa), 

 although its leaf-traces have only one bundle, is much more compli- 

 cated (Fig. 144), for the bundles maintain a distinct course throughout 

 twelve internodes before coalescing. Each bundle at first descends 

 in a straight direction through four internodes ; it then curves to the 



Fig. 143. — Diagram of the cour.se 

 of the vascular bundles in a 

 young branch of Juiiiperus nana 

 .shown on the unrolled surface 

 of the cylinder. At kk the 

 vascular bundles passing to the 

 axillary shoots are seen. (After 

 Gevler.) 



