130 



ANATOMY. 



PC 



ST 



formation of two concentric zones between the wood and bark ; it elongates at its 

 extremity only, while the stem and its branches elongate throughout their length ; 

 this may easily be proved by marking off an inch of a root and an inch of a stem. 

 Monocotyledons, instead of having a tap-root (i.e. one main axis which branches), 

 usually emit compound roots, i.e. composed of simple or slightly branched bundles, 

 rising from the neck. Their anatomical structure is exactly similar to that of 

 stems. 



Leaves. The anatomical structure of leaves is the same as that of the stem ; 

 they consist of a fibro-vascular bundle and parenchyma; this bundle, which is 

 wholly formed before leaving the stem, spreads into a blade as it emerges (sessile 

 leaf), or remains undivided for a certain distance before expanding (petiolate leaf) ; 

 the nerves of the blade are formed of fibres and vessels ; both it and the petiole are 

 covered with a layer of epidermis bearing stomata on every part except the nerves 

 and petiole. The petiole, before expanding, often forms a sheath or stipules ; 'the 



sheath exists when the 



partial bundles of which it 



is composed separate from 



each other, but without 



diverging; the stipules are 



the result of the diverg- 

 ence of the lateral bundles 



of the petiole. 1 



Where the fibro-vas- 

 cular bundle (fig. 696, FV) 



leaves the stem to form 



the petiole (F), the fibres 



composing it are shortened, 



and narrowed at each end, 



whence their surfaces O f uppcr e P illernii8 ; E ' Iower cpidermi8 - 

 contact are contracted ; they are hence not solidly united at the point of 

 emergence ; and it is this defective cohesion which causes the fall of most leaves. 

 The stem presents a little swelling at the base of the petiole, called the cushion 

 (c), which is visible after the disconnection of the petiole (fig. 54), together 

 with the scar (F) left by the petiole. The relative position of the elements of the 

 fibro-vascular bundle which passes from the stem into the leaf, shows clearly that 

 the leaf-blade may be compared to a flattened stem, the fibres and vessels of which 

 have been spread out, and thus allowed plenty of room for the development of 

 parenchyma between their ramifications. As in the stem the fibro-vascular bundle 

 consists of tracheae in the centre, then rayed or dotted vessels and woody fibres, 

 and on the outside laticiferous vessels and thick-walled liber-fibres, so in the leaf- 

 blade each nerve (which is a partial bundle) presents tracheae on its upper surface, 

 rayed or dotted vessels with woody fibre on its lower surface, and laticiferous 



PC 



FV M 



Branch cut vertically, 



Ri 



697. Melon. 



This theory of the origin and development of stipules requires considerable modification. ED. 



