SECT, i MORPHOLOGY 123 



in the length of the petiole, a, modification which facilitates the con- 

 duction of food material. They are often thickened and so arranged 

 as to meet the altered requirements for mechanical rigidity. 

 The vascular bundles pass from the leaf-stalk into the stem and 

 there either arrange themselves among the bundles of the central 

 cylinder or at once fuse with some of them. In the leaf-stalks of 

 Angiosperms the bundles usually appear arranged in a curve open 

 above, but may form other figures. In the petioles of Ferns, the 

 partial cylinders are accompanied, as in the stem, by sclerenchy- 

 matous fibres forming strands or plates. It is the peculiar arrange- 

 ment of those brown-walled sclerenchymatous masses which forms 

 the double eagle apparent on cross-sections of the petiole of Pteris 

 aquilina, from which the plant derives its specific name. 



In certain families of the Dicotyledons, particularly iu the Crassulaceae, the 

 mesophyll of the leaf-lamina forms peculiar masses of tissue called the EPITHEMA 

 between the swollen terminations of the bundles and the epidermis. The cells of 

 the epithema are small and, for the most part, devoid of chlorophyll ; they are full 

 of water, and joined closely together, leaving only very small interspaces, which are 

 filled with water. They are internal hydathodes (cf. p. 108) and serve for the active 

 excretion of water. Water pores 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 ( 118 ). 



The Course of the Vascular Bundles ( 119 ). The bundles exhibit 

 a definite course and arrangement within the body of a plant. 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 flower 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 pass 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 dis- 

 tinguished 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 down- 

 wards 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 traversed before the coalescence of the 

 bundles takes place. A relatively simple case is afforded by the 

 young twigs of the Dwarf Juniper (Juniperus nana) (Fig. 133). 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 



