388 MANUAL OF BOTANY 



upper surface (fig- 111) : hence is derived the term meristele. 

 The separate bundles have tlieir bast to"wards the lovs^er, and wood 

 towards tlie upper, face of the petiole. They have a common en- 

 dodermis and pericycle. In the second case there is a complete 

 ring, but the bundles are much smaller on its tipper side {fig. 771, 

 b). In this case the orientation is similar to that of the stem, the 

 wood being nearest the centre of the circle, and its development 

 centrifugal. 



When there are more meristeles than one in the phyllopodium 

 the central one is usually the largest. There are frequently 

 three, the two lateral ones being of small dimensions. 



The composition of the bundles is the same as that in the 

 stem froru which the leaf proceeds, being collateral, bicoUateral, 

 or concentric. 



In the petioles of Cycas the orientation of the bundles differs 

 from that of the stem, the protoxylem of the former being near 

 the exterior, so that the differentiation of the primary wood is 

 mainly, though not completely, centripetal. 



Secondary tliickening is very rare in the phyllopodium : it 

 does occur, however, in a few families, owing to the persistence 

 of a cambium layer in the usual position in the bundles. The 

 formation of secondary vascular tissue is but slight, and ceases 

 when the leaf has reached its full size. It is less in the limb 

 than in the petiole. 



The hypopodium in deciduous leaves is the seat of a meris- 

 matic formation which leads to the separation of the leaf from the 

 stem. A layer of cells extending completely across the phyllo- 

 podium, usually exactly at the base of the leaf-stalk, divides 

 several times so as to form a thin sheet of delicate thin-walled 

 cells, and by the absorption of the middle layer of the sheet the 

 two parts become separated and the leaf is cut off. The 

 ruptured surface is found to be covered with a layer of cork. 



The Leaf -blade. — When the epipodiiim of the leaf remains 

 unbranched, it is continuous in a straight line with the petiole 

 if the latter is present. The central axis of the leaf-blade 

 much resembles the petiole, but is rather more flattened. Its 

 lateral margins are continued outwards as a winged expansion. 

 The continuation of the petiole forwards constitutes the midrib, 

 and, as might be expected, the structure of the two is essentially 

 similar (compare figs. 772 and 773). 



The epidermal and cortical tissues are continued onwards 

 from the one to the other without any break, and if the petiole 



