DIV. I 



MORPHOLOGY 



95 



the leaves, and on the other hand conduct organic substances from the 

 leaves to the root system. The bundles are embedded in the other 

 tissues of the central cylinder and contrast with these owing to the 

 narrowness of their elements and the absence of intercellular spaces. 

 When the central cjlinder and cortex are sharply delimited by a 

 sheath the vascular bundles do not as a rule abut on this, but are 

 separated by a zone one or more layers thick (Fig. 112 A, B, pc) 

 which is called the PERICYCLE. 



The vascular bundles have a definite course and consequently a 

 special arrangement as seen in a transverse section of the stem. In 

 transverse sections of the internodes they appear arranged in a circle 



A 



. 112. A, Part of transverse section of a young stem of -Aristolochia Sipho. e, Epidermis ; pr, 

 primary cortex ; st, starch sheath ; c, central cylinder ; pc, pericycle, in this case with a ring of 

 sclerenchyma fibres ; ci/, phloem, and cv", xylem portions of the vascular bundle ; cb, cambium 

 ring; m, medulla ; mis, primary medullary ray. (x 48.) B, Small portion of the periphery 

 of a similar section of a still younger, stem, e, Epidermis ; pr, primary cortex ; st, starch 

 sheath with easily-movable starch grains ; pc, outer layers of the pericycle. ( x 350. After 

 STKASBURGER.) 



in the Horsetails (Equisetum) and most Gymnosperms and Dicotyledons 

 (Fig. 111/1). In most Ferns and in Monocotyledons (Fig. 109), on 

 the other hand, they are irregularly scattered. If the bundles form 

 a single circle (Fig. 112 A) the tissue within this, composed of 

 parenchymatous cells which are alive or may die at an early period, is 

 distinguished as the PITH (m). The tissue between the bundles forms 

 the MEDULLARY RAYS (ms). This distinction is wanting when the 

 bundles are scattered (Fig. 109). 



There are also Dicotyledons in which the vascular bundles form two (Cucurbita. 

 Phijtolacca, Piper] or more circles (Amarantus, Papaver, Thalidrum). The more 

 internal circles are usually less regular. 



The medullary rays may consist of parenchyma, but in a number of herbs their 



