DEVELOPMENT OP THE LEAP 



171 



grasses (a, in Fig. 465 A). Instead of passing around the stem, the 

 edges may come together between the leaf and the stem, so as to produce 

 a hollow tube, as in the Sarracenia. Let it next be assumed that the 

 apical portion, as well as the central-basal, enlarges, with little enlarge- 

 ment of the axial or lateral portions. We shall then get a form in which 

 a Lamina, or Leaf-blade, is superposed directly upon a Leaf-sheath. 

 Such a leaf, expanded, would appear as in Fig. 470, if the blade were 



46SA. 



46SB. 4 JO. 471. 



4r2 



Figures illustrating the origin and development of the parts of the leaf: Fig. 465 A. The grass-leaf: 

 a, the open sheath; b, the ligule. 465 B. The sedge-leaf: a, the closed sheath. 466. The primordial 

 leaf, a mere scale. 467. The same, as equally developed in all parts. 468. The same, divided into its 

 different regions: a, the apical portion; 6, basal portion; c and d, lateral portions with axial portion 

 between. 469. The same, undeveloped except the basal portion, which becomes a sheath to surround 

 the stem. 470. The same, with the apical portion also developed to form a blade, the lateral and 

 axial portions undeveloped. 471. The same, with the lateral portions developed into stipules. 472. 

 The stipules with their inner margins connate between the blade and stem, their outer connate around 

 the stem, forming an upper sheath or ochrea. In 465 A they are connate onlj'' by their inner margins, 

 between blade and stem, forming the ligule, {Adapted from work of A. A. Tyler.) 



but little developed, or it might be developed equally in both parts. 

 Both of these forms are frequently encountered. If now the lateral 

 portions shall enlarge, the axial portion not much elongating, a lateral 

 appendage must result at the base upon either side, as in Fig. 471. 

 These are the Stipules. If the stipules, instead of existing separately 

 in this way, shall incline together between the stem and the leaf, and 

 their inner edges cohere, it is clear that they must form a small blade 



