I 



978 



A TEXTBOOK OF THEORETICAL BOTANY 



The mature leaf consists of three portions, the lamina, the petiole, 

 and the leaf base or phyllopodium. In most Dicotyledons the leaf base is 

 relatively undeveloped, though it may in some cases give rise to the pulvinus, 



UPPER DEKMATOGEN 



■^- UPPER EPIDERMIS 



ADAXIAL LAYER — 



PROCAMBIUM 



ABAXIAL LAYER 

 LOWER DERMATOGEN 



■< 



PALISADE 

 PARENCHYMA 



VEINS 



LAYERS OF 



SPONGY 



PARENCHYMA 



LOWER EPIDERMIS 



UPPER DERMATOGEN- 



ADAXIAL LAYER- 



ABAXIAL LAYER 



< 



MIDDLE LAYER 



z: 



PROCAMBIUM- 



'ABAXIAL LAYER 



-*- 



LOWER DERMATOGEN • 



UPPER EPIDERMIS 



PALISADE 

 PARENCHYMA 



XYLEM AND 

 PHLOEM OF VEINS 



MIDDLE SPONGY 

 PARENCHYMA 



LOWER SPONGY 

 PARENCHYMA 



LOWER EPIDERMIS 



Fig. 969. — Diagram showing two schemes (A and B) of differentiation of the leaf tissues 

 from the marginal initials (Mi) and subepidermal initials (Si) respectively. A, Bougai?i- 

 riUea spectabilis. B, Carya bucklei. {After Adriaiice Foster.) 



a short cylindrical cushion at the base of the petiole, which acts as a joint 

 between leaf and stem, and by means of which the position of the leaf may 

 be changed. In other cases the phyllopodium gives rise to the stipules (see 

 p. 991). Many Monocotyledons, however, have greatly enlarged leaf bases, 

 forming sheaths surrounding the stem, which may, as in some Grasses and 

 Sedges, be the longest part of the leaf. In Palms with palmate leaves, such 

 as Chamaerops, moreover, there is ground for the belief that the petiole also 

 belongs to the phyllopodium, since the ligule, which in the Grasses and Sedges 

 occurs at the top of the sheath, is, in these Palms, at the top of the petiole, 

 immediately below the lamina (Fig. 970). 



These peculiarities are associated with the persistence of intercalary 

 growth in monocotyledonous leaves, which in many instances continue to 

 elongate throughout their life-period by the activity of a basal zone of meri- 

 stematic cells. An analogous persistence of growth at the base of the lamina 

 in other cases leads to the development of cordate or sagittate leaves with 

 outstanding basal lobes, or in extreme cases to peltate leaves in which there 

 is an extension of the lamina below the point of junction with the petiole, 

 which then appears to be inserted in the middle of the lamina. 



The suppression of laminar growth at certain points, which may also 

 occur, gives rise to the irregularities of outline which we have previously 



