SUNFLOWER. STEM. 53 



The development of the resin-passages may be observed with 

 great ease and certainty in transverse sections of the stem of Ivy 

 (Hedera Helix). Cut transverse sections from a young succulent 

 stem, mount in glycerine. Scattered through the cortex and pith 

 will be found passages already well developed, and having a 

 structure similar to those in Helianthus. If the soft bast, which 

 lies immediately outside the cambium, be examined carefully, 

 resin-passages will be found in various stages of development, 

 starting from a group of four cells, with no intercellular space. 

 In older stages the cell-wall will be found to have split at the 

 angle where the four cells meet, while in older stages again the 

 intercellular space appears larger ; meanwhile divisions (radial 

 and tangential, the former more frequent) occur in the epithelial 

 cells. 



Note that in (1), (2), and (3), there occur, especially 

 in stems growing apace, divisions of the cells in a 

 radial direction. Compare the girth of the stem at 

 the upper with that at the lower part of the plant, 

 or that of a young plant with that of an old one. The 

 conclusion will naturally be drawn that the stem 

 increases in girth as it grows older, and since the outer 

 tissues neither peel off, nor do the individual cells 

 increase greatly in width, longitudinal radial divisions 

 of the cells are the only alternative. 



Before leaving the cortical tissue it must be noticed 

 that the Bundle-sheath, which is the inmost layer 

 of the cortical tissue, and which is easy of observation 

 in the younger stem (cf. Hypocotyledonary stem) may 

 be identified also in these sections, though with diffi- 

 culty. The layer of thin-walled cells abutting directly 

 on the thick- walled sclerenchyma fibres (yellow with 

 Schulze's solution) show in their radial walls the char- 

 acters of a bundle-sheath i.e. (i.), they are coloured 

 brown with Schulze's solution; (ii.), they resist the 



