THE GROWTH OF BOOTS AND STEMS. 171 



consist chiefly of long tubes, called sieve-tubes, because at intervals each 

 is crossed by a partition which is perforated like a sieve, so that the 

 contents (which are proteid substances) can pass through the pores in 

 the sieve-plates from one part of the tube to another. The sheath 

 consists chiefly of two layers of cells (endodermis and pericycle in 

 Fig. 58), but these two layers are not very sttarply marked in the Bean 

 root. The pericycle is a very important layer, because its cells remain 

 capable of dividing and giving rise to new cells ; from the pericycle 

 arise (1) the rootlets, (2) part of the new wood and bast formed as the 

 root grows older and thicker, (3) the cork which covers the surface in 

 the older parts of the root. 



(c) In roots of Cress, Mustard, etc. , mounted whole and stained with 

 aniline chloride or red ink, also in sections through roots of Bean or 

 Pea, notice the formation of rootlets as small swellings (arising from the 

 pericycle) opposite the wood-bundles ; these swellings grow out, eating 

 their way through cortex and skin-layer, and escape as small pegs 

 which grow into rootlets. The rootlet is enabled to digest the tissues 

 through which it bores its way, by the cells covering its tip producing 

 ferments similar to those found in germinating seeds. Do you see why 

 rootlets are arranged in a definite number of longitudinal rows? 



(d) In older parts of the root of the Bean and other dicotyledons 

 new tissues are formed, and the root grows thicker. This is because 

 the cells of the sheath (pericycle) outside each wood-bundle, together 

 with the cells of the central cylinder which lie inside each bast-bundle, 

 divide actively and give rise to new bast and new wood. 



Cut cross-sections of old parts of the main root of Bean, stain with 

 aniline chloride, and notice the new wood and bast that have been 

 formed ; the original wood-bundles (primary wood) can still be seen in 

 the centre of the root. 



(e) In sections of the root of Maize or other monocotyledons notice 

 the large number of wood and bast-bundles ; no secondary thickening 

 occurs in these roots. 



201. Continuity of Tissues of Root, Stem, and Leaf. 



It is very important to realise that the tissues (epidermis, 

 ground-tissue, vascular tissue) of the root, stem, and leaves 

 are continuous with each other. 



The epidermis of the leaf and of young stems is a pro- 

 tective layer, preventing excessive transpiration for instance, 

 and where necessary it bears a layer of cuticle or of waxy 

 " bloom," or a covering of hairs, to help in this function ; 

 another important function is to allow of the passage of 



ses into and out of the plant, by means of the stomates. 

 he epidermis of the younger parts of the root serves for 

 absorption, and to increase the absorbing surface root-hairs 

 are developed. In the older parts of the stem and root 



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