ENDS AND CONNECTIONS OF THE BUNDLES. 



373 



Chlorophyll. Its elements, in so far as they stand in immediate connection with the 

 tracheae, approach on the one hand the latter, on the other hand the typical paren- 

 chymatous cells in their form. This is the rule for the leaves of Monocotyledons and 

 Dicotyledons ; the exceptional case in which even the ultim.ate transverse branchlets 

 are completely enclosed by stout 



SsS^S^^?** 



FIG. 174. — Zea Mais. Cross-section through a feeble (lower) leaf- 

 sheath, about 2 cm. higher than Fig. 151, p. 331. e e[)idermis of the 

 outer surface, bordering internally on a hypodermal bundle of scleren- 

 chymatous fibres, sf. On the inside of the latter one of the smaller lon- 

 gitudinal vascular bundles abuts ; ?' annular vessel, .v air-cavity, i — i 

 pitted vessels ; outside /, r, t are first narrow reticulated and pitted 

 vessels (with a darker outline), then the phloem yv ; q transverse 

 branchlet springing from the tr.ichere of the vascular bundle and 

 consisting only of a few rows of tracheides. 



sclerenchymatous sheaths occurs 

 rarely in thick leaves of Mono- 

 cotyledons, e.g. Rhapis, Vanda 

 furva. In the leaves of Ferns the 

 branches of the bundles are, so far 

 as investigated, always ensheathed 

 by one or a few layers of elongated 

 cells destitute of chlorophyll ^, ot 

 which the outermost often has the 

 structure of an endodermis up 

 to the immediate neighbourhood 

 of the free ends. At the free 

 ends themselves the rows of 

 tracheides pass over into the 

 chlorophyll-parenchyma, through 

 the intervention of some elon- 

 gated smooth-walled cells. 



In the case of many ends of 

 bundles, which according to their 

 local position must be called peri- 

 pheral, no essential differences 

 from the internal ends are to be 

 mentioned, On the other hand, 

 the structure shows peculiarities in 

 ♦hose numerous cases, where the 

 bundles run to parts of the epi- 

 dermis described in Chap. I, which 

 are distinguished by water-pores 

 and water-filtration, by excretion 

 of lime, or by glandular structure 

 and secretion. 



Of the cases belonging to 

 this series, in the first instance, the 

 ends of the bundles in the furrows 

 of Fern-leaves excreting water and 

 lime (p. 106) are closely similar to 

 the internal terminations in these 

 leaves'^. They show a knob-like swelling, in consequence of a sudden increase in 

 the number and size of the tracheides, the latter being very short, with narrow 



Fig. 175.— Cross-section through the lamina of the leaf of a young plant of 

 Zea M^is. epidermis of the upper, u of the lower surface ; h hypodermal 

 strands of sclerenchjTna ; gx and g2 two small longitudinal vascular bundles in 

 cross-section; g\ with three, ^2 with two narrow vessels; both with a small 

 phloem, consisting in g2 of only three elements ; gT, transverse connecting 

 branch between g\ and g^, consisting of a row of tracheides with partial 

 fibrous thickening, and, like the two longitudinal bundles, enclosed directly 

 in parenchyma containing chlorophyll. 



^ Cf. Mettenius, Fil. hort. Lips. p. 9. 



- Mettenius, /. c. 



