THE ANATOMY OF THE SHOOT 83 



inhibition of the marginal meristem proceeds from the apex toward 

 the base. The activity of the plate meristems consists essentially 

 of anticlinal divisions in the respective layers so that no pro- 

 nounced thickening of the blade results except in the region of 

 the procambial strands. Differential rates of cell division and cell 

 enlargement occur in the various tissues of the developing leaf. 

 The palisade cells continue division after the cells of the spongy 

 parenchyma have stopped dividing, and cell enlargement usually 

 continues longest in the epidermal cells. These factors create 

 stresses v^hich account for the schizogenous formation of air spaces 

 in the mesophyll. 



The significant stages in the ontogeny of a simple foliage leaf 

 may be summarized as follows : 



(i) The development of a meristematic primordium at the foliar 

 buttress. 



(i) The extension of the primordium by apical and intercalary 

 growth to form the petiolar and midrib region. 



(3) The development of the lamina by marginal meristems. 



(4) The differentiation of the tissues of the lamina from marginal 

 and submarginal initial layers — protoderm, ground meristem, 

 and procambium. 



(5) The surface growth of the lamina owing to the activity of 

 plate meristems. 



Ontogeny of Compound Leaves. — The foregoing account is 

 based upon the development of simple leaves. The situation with 

 respect to the differentiation of compound leaves has been less 

 thoroughly investigated. In general, the leaflets or lobes of leaves 

 arise in basipetal succession, from the apex toward the base; in 

 acropetal succession, from the base toward the apex; or in what 

 Troll (40) has termed a divergent sequence in which intermediate 

 leaflet primordia first develop, and additional leaflets form both 

 acropetally and basipetally from that point. An example of the 

 acropetal method of development has been described by Sachs (31) 

 for the Umbelliferae. In this instance, the initials of the leaflets 

 arise at the base of the cone-like primordium of the leaf, and grow 

 apically in the same manner in which the central portion of the 

 leaf primordium develops. Secondary leaflets are formed in a 

 similar way in cases where the leaf is bipinnately compound. 

 (Fig. 31.) Foster (16, 17) has made a study of the ontogeny of 

 the leaf of Carya in which the development is also acropetal. 



