94 



MORPHOLOGY OF TISSUES. 



plants possess closed bundles, or, if they are open, the activity of their cambium 

 soon ceases. 



The different forms of tissue of a differentiated fibro-vascular bundle may be 

 classified into two groups, which Nageli calls the Phloem- (Bast) and Xyle7n- (Wood) 

 portion of the bundle. They are separated by the cambium if there is any. In many 

 bundles the phloem is formed on one, the xylem on the other side of the procam- 

 bium, and the development of both advances towards the centre of the bundle, where 

 at length they meet. The phloem consists of succulent, generally thin-walled cells ; 



Fig. 8i.— Transverse section of a closed fibro-vascular bundle in the stem of maize (X550) ; fip the surrounding thin-walled 

 parenchyma ; a outer side, i inner side (facing the axis of the stem) ; g g two large pitted vessels ; s spirally thickened 

 vessel ; r isolated ring of an annular vessel ; / air-containing cavity, from splitting caused by growth ; •z/z- the cambiform or 

 latticed cell-tissue which has passed over last into permanent tissue ; between it and the vessel j lie reticulately thickened 

 and bordered pitted vessels ; the periphery of the whole bundle forms a firm sheath of thick-walled lignified prosen- 

 chyma-cells. 



only the bast-cells, which are often absent, but very often massively developed, are 

 usually greatly thickened (mostly however not lignified but flexible). These thin-walled 

 succulent cells are either parenchymatous, or they are cambiform or latticed-cells, or 

 finally sieve-tubes. The xylem-portion of the fibro-vascular bundle has mostly a 

 strong tendency to thicken its cell-forms ; their walls become hard and woody ; 

 in vessels and the bordered pitted wood-cells the contents disappear, and they 

 henceforth conduct air. Woody parenchyma is also abundant, but in some cases 

 the lignifying does not take place; the whole bundle is then soft and succulent, 

 sometimes traversed only by single thinner bundles of lignified vessels and wood-cells 



