THE TISSUES OF THE STEM 45 



which lies peripherally, is called the phloem, or bast (phi) ; a firmer 

 region, which lies next the pith, is called the xylem or wood (xy). 

 Between them are very thin-walled cells showing active division ; 

 this is the cambium (c). The transition between this and the wood 

 appears sudden in the drawing ; but the radial rows in which the 

 cambial cells are arranged may be traced inwards into the wood, 

 and also outwards into the bast, showing that all the tissues are 

 structurally related. 



The most marked constituents of the xylem or ivood are radial rows 

 of tracheae, or vessels which are elements with thick woody walls. The 

 smallest are nearest to the pith ; these are the protoxylem (px), or 

 first-formed tracheae of the wood. The larger tracheae are pitted 

 vessels (p.v.), and the transition from one to the other is gradual. 

 There is no protoplasm in any of these, but they are embedded in 

 tissue of which at least some cells retain it, and are thus alive. 

 Near to the protoxylem the tissue is thin-walled, but towards the 

 cambium it is thick-walled and woody, so that here the xylem forms 

 a firm and coherent mass. The phloem or bast has no woody walls, 

 but is soft, with cellulose walls and protoplasmic contents. The most 

 marked constituents are the rather wide sieve-tubes (s), which are em- 

 bedded in thin-walled parenchyma. A layer of more or less definite 

 cells, called the pericycle (p, p) adjoins the endodermis internally. 



A very incomplete idea is obtained by seeing the tissues only in 

 transverse section. It is like seeing the ground plan of a building 

 without the " elevation." The tissues must also be examined in 

 longitudinal section, so that each unit is followed throughout its 

 length, and the character of its lateral walls disclosed. If a longitu- 

 dinal section be taken in a radial plane through the stem of the Figwort, 

 so as to cut through the middle of a vascular strand like that in 

 Fig. 23, its appearance would be as in Fig. 24. To the right are the 

 external tissues, and the pith to the left. Starting from the epidermis 

 (epid) its cells are oblong, with their outer walls thickened and cuticu- 

 larised. A stoma has been cut through longitudinally (arrow). Below 

 is a thin-walled cortex, with intercellular spaces. There is no collen- 

 chyma here, and the cortex is a narrow band. It is limited internally 

 by the closely-fitting oblong cells of the endodermis (endod), with 

 numerous starch-grains, which marks off the cortex from the stele. 

 The vascular tissue shows up as a compact strand of more elongated 

 elements, the phloem being thin-walled, and the xylem thick-walled 

 and woody. The sieve-tubes are now seen to be long cylinders, tra- 

 versed here and there by oblique septa of beaded appearance. 1 hese 



