154 



ACADEMIC BOTANT. 



grows, the wood-bundles nearest this central tissue often cohere, and 

 the central tissue disappears, making the 

 stem hollow, as in the grasses. Some- 

 times it persists at certain distances, 

 making joints, or stories, as it were, to 

 strengthen the walls of this aerial house, 

 as in the cane and bamboo. 



378. Exogenous stems (Fig. 81) are dif- 

 ferentiated into pith, wood, and bark ; the 

 pith (L. medulla, marrow) in the centre; 

 the wood next outside the pith ; the bark 

 next outside the wood ; the epidermis next 

 outside the bark. Though cellular at first, 

 it soon exhibits the order which character- 

 izes it (Pig. 224). Pibro-vascular bundles 

 (wood) appear in regular wedges pointing 

 towards the centre, which remains cellular 

 (pith, M). The spaces between the wood- 

 wedges are the same as the pith ; they are 

 the medullary rays (KM). The black 

 inner portions of the wood-wedges are the tracheae (T) ; they become 

 the m,edullary sheath; these tracheae, which are the spiral vessels (Fig. 

 220, A), are usually found nowhere else in the plant. They are the 

 first vascular tissue in the stem. The dark outer portions of the 

 wedges are the fibrous tissue of the inner bark. As the stem grows, 

 the wood-wedges enlarge, their tracheae form a sheath around the pith, 



and the medul- 

 lary rays con- 

 tract into nar- 

 rower lines, as 

 in Fig. 81, but 

 still keep up 

 c mmunication 

 between the pith 

 and the bark. 



Fig, 224. — Horizontal section 

 of young Melon stalk iCwumbis 

 Meto) : M, medulla, or pith ; HM, 

 medullary rays; T, tracheae, or 

 spiral vessels, forming the med- 

 ullary sheath. The points along 

 the circumference are hairs on 

 the epidermis. 



KiG. 225. — A, section of stem of Rice-paper-tree (Falitta papyrifera): p, pith; 

 central cavity loosely filled witli large, round cells, of which 3 are seen ; c, 

 large central pith-cells ; uj, wood ; 6, bark ; e, epidermis. B, same, pith sepa- 

 rated from wood, showing a partial cavity in the centre (o) iilled with large 

 cells. C, a slice of the pith as it appears in the paper. 



379. The Pith in all growing parts is full of juices. As 

 the parts mature, the pith is emptied ; it becomes dry and 

 light, containing nothing but air, and is of no further use 



