42 



Elementary Botany 



portion, leaving a band of meristem (cambium) between them 



(fig- 6S). 



The bundles, which now consist of fibres and vessels, with 

 a few parenchymatous cells, are spoken of as fibro-vascular 

 bundles. In many cases the fibres and vessels become very 



much hardened by the 

 thickening of the second- 

 ary deposit, and the fibro- 

 vascular bundles can then 

 be easily separated from 

 the surrounding portions 

 of the stem. In other 

 cases there is very little 

 hardening in the bundles, 

 and they cannot be thus 

 separated. 



The rest of the stem in 

 its young state is made up 

 of parenchymatous cells, 

 forming what is known as 

 fundamental or ground 

 tissue, which is divided 

 into three parts — a portion 

 in the centre of the stem, 

 the pith or medulla ; a ring 

 underlying the epidermis, 

 from which the cortex or 

 bark is developed ; and, 

 lastly, masses of cells 

 which separate the fibro- 

 vascular bundles and unite these two portions, the medullary 

 rays. 



The next change is in the cells which lie between the fibro- 

 vascular bundles and the epidermis. Here are produced 

 flattened cells filled with air, possessing flexible and elastic 

 walls forming a close tissue without interspaces. These are 

 cork cells. They are rarely (as in the Willow) developed from 

 the epidermis itself; sometimes (as in the Poplar) from the 



Fig. 65.— Transverse section through a young 

 internode of Bcchmcria argentea : o, epider- 

 mis : CO, outer cortex (coUenchyma) ; R, inner 

 cortex : s, intercellular space \ c, cambium of 

 the vascular bundle ; c', cambium of the thick- 

 ening ring ; b, bast portion ; H, xylem portion 

 of the vascular bundle ; b', the bast formed 

 from the intermediate cambium ; 2, medullary 

 ray. 



