STRUCTURE OF COLLATERAL BUNDLES. 



33^ 



^^^^^^S 



the leaf and the upper part of the leaf-sheath the bundles are similar to that in Fig. 130, 



though on the average smaller. 



Fig. 151 (i4s), on the other hand, is 



taken from the place where the bundle 



passes through the basal portion of the 



leaf-sheath of a young plant. The bundle 



itself, with its group of vessels at g, is in 



all parts smallerthan that in the preceding 



figure, but otherwise similar to it, as is 



clear without explanation, even in the 



finer points of structure not represented 



in the figure. Here however there is a 



single-layered sheath, consisting of deli- 

 cate parenchymatous cells, square in 



cross-section, around the whole bundle. 



Outside this, and separated by it from the 



phloem, is the thick strand of coUen- 



chyma, sc. Both the latter and the bun- 

 dle are surrounded by the parenchyma- 

 tous layer st, which is rich in starch (the 



starch-sheath. Sect. 122). e-e Epidermis 



of the outer surface. 



Fig. 152. Cross-section through a 



vascular bundle of the internode of a 



creeping stem (runner) of Ranunculus 

 repens (225). xx annular and spiral ves- 

 sels at the inner edge, t pitted vessels in 

 the external region of the xylem ; be- 

 tween and around the latter delicate- 

 walled elongated parenchymatous cells. 

 s phloem ; the larger meshes are sieve- 

 tubes, the smaller ones, partly dotted, 

 are cambiform cells. At 

 the inner border of the 

 phloem are delicate cambi- 

 form cells in rows. Ex- 

 ternally to its constantly 

 thin-walled peripheral ele- 

 ments, the bundle is sur- 

 rounded by a thick scle- 

 renchymatous sheath, which 

 is only interrupted on the 

 outer border of the xylem. 

 Outside this is large-celled, 

 thin-walled parenchyma. 

 The longitudinal section 

 through the bundle, if not 

 quite accurately median, 

 would be similar to that 

 of Saururus, represented 

 in Fig. 57, p. 157. 



Fig. 153 (225) represents 

 the cross-section of one of 

 the larger bundles in the 

 internode of Ranunculus 



. Fig. IS2,. 



Fig. IS". 



