ALLIUM CEPA 197 



only the base of the sheath. The lowest portion of a half-grown blade 

 remains meristematic until the leaf is nearly grown, however, so that 

 this region gives rise to a large share of the blade. Any one leaf, 

 after it reaches a length of three centimeters, attains its mature length 

 in a few days." 



In the development of the leaf, mitoses, indicating continued cell 

 divisions, may occur in all parts, even after intercellular spaces are 

 formed. Transverse divisions and growth of the surrounding 

 parenchymatous cells cause the intercellular spaces to become en- 

 larged and much elongated throughout the entire leaf. There is a 

 peripheral zone in which the cells enlarge radially rather than 

 longitudinally; and, in the mature leaf, these form two or three 

 subepidermal layers of columnar cells which are comparable to the 

 palisade region found in dicotyledonous leaves. (Fig. 91, B.) 



The parenchymatous cells centrad to the chlorenchyma do not 

 keep pace with the growth of the latter, and a large central cavity 

 develops which extends through the entire length of the blade. 

 (Fig. 91, A.') Prior to the formation of the cavity, the inner cells 

 are alive, although the volume of the intercellular spaces formed 

 between them is probably greater than the volume occupied by the 

 cells themselves. Newcombe (ix) has described cavity formation 

 in onion and other plants, and regards the initial stages which 

 occur during the primary growth of the leaf as being schizogenous. 

 The final ones are lysigenous, and result from the death and collapse 

 of the centrally located parenchyma cells which by their disinte- 

 gration add to the size of the central cavity. 



The Mature Leaf. — The blade of the mature leaf consists of 

 the epidermis, the palisade layers, the vascular bundles, and eight 

 or ten rows of spongy parenchyma which surround the central 

 cavity. (Fig. 91, B.) The development of the xylem in the pro- 

 cambial strand is endarch, and differentiation proceeds rapidly from 

 the base of the leaf to its tip so that the first protoxylem elements 

 are mature throughout the entire length of the leaf before any of 

 the metaxylem is completely developed. Nearly all of the primary 

 xylem elements have spiral thickenings. 



The cotyledon has a single vascular bundle, and the number 

 increases in successive leaves, the second leaf having five bundles, 

 the third usually eight. Each of the older leaves has two more 

 bundles than the preceding one until, in the sheath of a mature 

 bulb scale, there may be 40 or more. The bundles are arranged 



