8o 



STEM OF CUCURBITA 



long, apparently empty, sieve-tubes (s.) and the narrow, densely 

 granular, companion cells (c.c.) are readily distinguished. The 

 vessels of the metaxylem (p.v.) bear very dense spirals or occa- 

 sionally bordered pits, whilst in the protoxylem the thickening 

 takes the form of rings or a loose spiral (px.). The narrow 

 elongated elements sometimes seen between are the wood- 

 parenchyma cells. 



Further insight into the structure of the Dicotyledonous stem 

 will be afforded by a study of that of the Vegetable Marrow 



FIG. 36. Longitudinal section through a vascular bundle of the Sun- 

 flower (Helianthus], c., cambium; c.c., companion cells ; Co., cortex ; 

 /..fibres of pericycle ; p., pith-parenchyma ; p.v., pitted vessel of meta- 

 xylem ; px., spiral vessel of protoxylem ; s., sieve-tube ; sh., starch- 

 sheath. 



(Cucurbita) (Fig. 37). The transverse section differs from that 

 of the Sunflower in the hollow pith, in the presence of a broad 

 ring of sclerenchyma (Scl.) some little way beneath the epidermis, 

 and in the arrangement and construction of the vascular bundles. 

 A pith-cavity is a frequent feature in the stems of herbs (e.g. 

 Labiatae, Umbelliferae, Gramineae, etc.), and can be related to 

 cessation in the growth of the pith-cells, so that rupture occurs 

 as a result of the continued enlargement of the stem. 



In the stem of the Marrow there are two rings of vascular 

 bundles, of which the larger and inner alternate with the smaller 



