132 IX. THE DICOTYLEDONOUS STEM HERBACEOUS TYPES. 



arranged, bright cells ; then the bast (cb)\ which appears some- 

 what less bright, and is also devoid of the regular arrangement 

 shown by the cambial cells, and which is accompanied 

 externally by a layer of brighter ground - tissue cells which 

 are poor in cell-contents. In somewhat thicker sections, and 

 with a low power, this layer is very distinguishable by its clear- 

 ness, in contrast with the surrounding, somewhat chlorophyll- 

 containing tissue. The white ring, next following outwards, is 

 composed of strongly-thickened sclerenchymatous elements ; 

 between the vascular bundles it projects somewhat inwards. 

 This sclerenchyma ring serves for the vascular system, as a 

 whole, the same mechanical purpose as is served by the scleren- 

 chyma sheaths of the individual bundles in Ranunculus. Next to 

 this ring, and farther outwards, is a tissue containing chlorophyll, 

 the innermost layer of which, bounding the sclerenchyma ring, 

 is marked by its richness in starch, and belongs to the category 

 of so-called "starch sheaths". Such a starch sheath corre- 

 sponds to the " endodermis " of roots, which we shall study 

 hereafter, and forms the innermost layer of the primary cortex. 

 The whole axial portion, enclosed within the starch sheath, is 

 the central cylinder or stele ; to it the sclerenchyma ring 

 belongs, and to that peripheral portion of it, the pericycle, to 

 which the parenchymatous tissue immediately outside the vas- 

 cular bundles also belongs. After treatment with iodine, or 

 chlorzinc iodine, the starch sheath stands out very clearly. To 

 the inner chlorophyll-containing tissue of the primary cortex 

 there follows a tissue, likewise to a small extent chlorophyll- 

 containing, and with narrow cavities, with glistening cell-walls, 

 more strongly thickened in the angles, which, by this last 

 characteristic, we know to be collenchyma a tissue generally 

 appearing like a glistening white sheet pierced by rounded holes, 

 and exceedingly common as a hypodermal tissue in Dicotyledons. 

 Outside all is found 'the epidermis. The collenchyma ring is, 

 however, interrupted by certain prolongations of the inner cortical 

 parenchyma, rich in chlorophyll, which extend up to the stomata. 

 The Cambium Ring. Having obtained this general informa- 

 tion, we will now turn to our specific object, namely, to determine 

 in what fashion the complete cambium ring of Dicotyledons is 

 produced ; and first we will examine a single bundle (Fig. 49) 

 under a higher power. The cross-section must be sufficiently 

 thin, and will be best prepared from alcohol material, and studied 



