THE ANGIOSPLRMAE : LEAN'ES 



979 



mentioned as characteristic of certain species of Artocarpus or Broimonetia. 

 In some of the Araceae it may affect, not only the margins, but the central 

 parts of the lamina, giving rise to characteristic perforations which are seen 



Fig. 970. — Development of the phyllopodium as a leaf 

 sheath. A, Aretw planiculmis. B, Heraclcttm 

 sphoudyliiim. C, Pontederia lordata. The leaf sheath 

 terminates upwards in each case in ligular out^'rowths. 

 (A and B after Velenovsky ; C after Arber.) 



in their most striking development in the large leaves of the tropical climbing 

 Aroid, Monstera deliciosa (Fig. 971). Patches of meristematic tissue in the 

 leaf rudiments of this plant die and drop out at a very early stage of develop- 

 ment and the margins of the lacunae are covered by a secondary' epidermis 

 formed from interior cells of the young leaf. 



The early arrest of growth in the midrib, associated with continued growth 

 of the lamina on each side of it may produce either an emarginate leaf such 

 as that of Liriodendron, or in extreme cases, an apparent dichotomy of the 

 leaf, which is characteristic of certain genera, such as Baii/iinia, a leguminous 

 tropical climber (Fig. 972). Leaves of this type may indeed have originated 

 from compound leaves, the arrested growth of the rachis leading to the 

 coalescence of opposite pinnae at their bases, but the principle of 

 development is the same in either case, namely arrest of the principal 

 axis, associated with extended lateral growth. This is not to be con- 

 fused with true dichotomy of the leaf blade which may sometimes occur as 

 an abnormality, for example in Sah'x. 



Palmatifid and pinnatifid leaves may, on the other hand, originate from 

 the opposite type of growth distribution, namely, the greater rate or longer 

 continuance of growth in and close to the main veins, with the result that 



