THE GROWTH OP BOOTS AND STEMS. 169 



form supporting tissue (collenchyina) which differs from 

 ordinary hard tissue in consisting of living cells. 



The ground-tissue is also modified in places to form scleren- 

 chyma, consisting of thick- walled woody fibres, which contain 

 no living substance but only water, and which, therefore, serve 

 a purely mechanical function. The sclerenchyma is found 

 outside of the bundles, either forming (1) a complete tube 

 around the whole stem, or (2) a strand on the outer side of 

 each vascular bundle, or (3) a sheath around each bundle 

 (what plants afford examples of these three arrangements ?). 

 The sclerenchyma of the stem is continued into the leaf, 

 where it usually forms a supporting band below each chief 

 vein or above it or in both positions. The ground-tissue also 

 serves for the conduction of sugar from the leaf, and for the 

 storage of starch. In Dicotyledons the new strips of cambium, 

 which are required to form a complete ring of growing tissue 

 around the stem, are formed in the regions of ground-tissue 

 (rays) which lie between the vascular bundles. 



(3) The most important structures in the wood and the 

 bast are the wood- vessels and the sieve-tubes ; the former 

 serve to carry water upwards from the roots, the latter to 

 carry organic substance, especially proteids, to the growing 

 parts of shoot and root. The wood serves also as mechanical 

 (supporting) tissue; it usually contains fibres and ordinary 

 cells (parenchyma) in addition to vessels. 



20O. Structure of the Boot. The root, like the stem, 

 contains vascular bundles, but the wood-bundles are separate 

 from the bast-bundles and lie on alternate radii, whereas in 

 the stem the wood and bast are joined to form each bundle. 

 The vascular cylinder of the root contains from two to about 

 eight bundles of wood and an equal number of bast-bundles 

 in Dicotyledons, while in Monocotyledons the number of 

 bundles is generally much larger. 



* (a) Place the root of a Bean or Pea seedling (try others as well) in 

 some water on a slide, and scrape off the soft outer tissue (epidermis 

 and cortex), first on one side and then on the opposite side. Wash off 

 the scrapings (using your wash-bottle), place on the scraped root some 

 aniline chloride solution, add a drop of glycerine (which makes tissues 

 more transparent), put on a cover-glass, and examine. Notice the 

 yellow-stained wood- vessels, and the long cells associated with them ; 

 sketch what you see. 



