106 VIII. THE MONOCOTYLEDONOUS STEM. 



walls of these ground-tissue cells in the fully developed stern are 

 also coloured yellow by the chlorzinc iodine, and only here and 

 there show a shade of violet. 



Approaching now the periphery of the stem we note that the 

 vascular bundles become much more closely crowded together, 

 their cross-section is smaller, the intercellular passage disappears, 

 and a median row of vascular trachei'des takes its place. In the 

 outermost smallest vascular bundles both protoxylem and proto- 

 phloem are wanting, which is connected with the fact that these 

 vascular bundles are first formed after the elongation of the inter- 

 node has ceased. The two lateral vessels diminish in size ; they are 

 connected by others placed between them. The bast undergoes a 

 specially marked reduction. On the other hand, the scleren- 

 chyma sheath of 'the bundle increases in thickness, but only 

 around the inner portion of the vascular bundle. In proportion 

 as the sclerenchyma sheath increases in strength, the so-called 

 " transfusion " places at the limits of the wood and bast show 

 more and more clearly. At these places the vascular bundle 

 sheath remains feebly developed, its elements unthickened. The 

 transfusion places permit the exchange of nutrient materials 

 between the interior of the vascular bundle and the surrounding 

 ground-tissue. In the outermost vascular bundles, which have 

 undergone the greatest reduction, the bast is seen to be sunk in 

 the wood ; the two transfusion places have joined on the outer 

 side of the bundle, and form one common transfusion place ; 

 while the sclerenchymatous elements of the sheath are altogether 

 wanting there. 



Outside the central cylinder lies the primary cortex. Next to 

 the epidermis of the stem comes a more or less strongly-developed 

 ring of tissue, the elements of which have the same appearance 

 as those of the vascular-bundle sheath, and show the same reac- 

 tion with chlorzinc iodine. Such a distinct sheath of tissue bound- 

 ing the epidermis is known as Hypoderma. This hypoderma is 

 interrupted only under the spots where lie the stomata. The 

 hypoderma and the sheath of the vascular bundle have alike to 

 provide for the protection of the thin-walled tissue and for the 

 general stability of this part of the plant, and are included amongst 

 the elements of the mechanical system, as Stereides, while the 

 tissue which they constitute forms the mechanical tissue system, 

 the Stereome. In proportion as the stem needs to be secure 

 against flexion, so must the mechanical appliance, the stereome, 



