4o6 PRIMARY ARRANGEMENT OF TISSUES. 



leaf, the pulvinus of the Cyatheacece, tlie air-containing parenchyma appears on the 

 sides and on the dorsal surface in the form of round or oblong, sharply circum- 

 scribed, isolated groups, with a breadth and depth of a few millimetres, which con- 

 sist of a mass of cells shaped like many- rayed stars, leaving between them wide 

 lacunae, and covered by a portion of epidermis containing stomata. At first narrow in- 

 terstices containing air load from the lacunae into the deeply-seated tissue. At a very 

 early stage of development, in the cases investigated before the petiole and lamina 

 begin to unroll and to unfold, ,the epidermis and the mass of stellate cells covered 

 by it die off, while the walls of the latter become thickened, and assume a yellowish 

 brown colour ; the dead and crumbling mass leaves a furrow behind, filled by its 

 powdery remnants, and this becomes sharply limited owing to the sclerosis of the 

 surrounding multiseriate cellular layer. The latter becomes attached, all round the 

 petiole, to the tough, sclerotic, brown, peripheral cortical layer, and appears in 

 the matvu-e condition as an indented portion of the latter. Comp. Figs. 140 and 141 

 (p. 292), or Fig. 189 aty. 



We need not enter here into further details of structure of petioles and ribs, 

 but may refer to the next chapter for some special points still to be brought forward. . 



Sect. 121. The space in the lamina of the /^o/" which is left free by the ribs and 

 vascular bundles, is mainly occupied by parenchyma, which is simply called leaf- 

 parenchyma or in the special case of flat foliage-leaves Diachyma or Diplo'e 

 according to Link '■^, Mesophyll according to De Candolle ^. In the little-developed 

 scales and cataphyllary leaves, especially those which are destitute of chlorophyll, it 

 shows no general anatomical peculiarities worthy of remark. In the case of the 

 green foliage-leaves special phenomena of structure and arrangement are known, 

 especially since the works of Treviranus * and Brongniart ^ Organs of a different 

 morphological value, which in certain plants assume the functions of foliage- 

 leaves — Phyllodes, Phylloclades, Halms, &c. — behave essentially hke leaves as 

 regards the structure of their parenchyma, as has already been to some extent 

 noticed above ; for this reason they are likewise to be discussed here. 



The parenchyma of the organs in question is, in great part at least, chlorophyll- 

 parenchyma^, in the sense of the word indicated at p. 116 ; its character and distribu- 

 tion primarily determine the conditions which are to be considered here. With 

 reference to these, two main types are to be distinguished, though they are united by 

 intermediate forms. 



I. In the first type, which may be called the cenlric, the chlorophyll-parenchyma 

 is uniformly distributed around the entire organ— if intercalated bands of non- 

 equivalent tissue be left out of consideration. In flat horizontal parts slight 

 differences occur in relation to the upper and lower side. 



To this type belong the leaf- like branches and ' Halms,' flat foliage-leaves 

 which are not horizontally placed, and many which are both flat and horizontal. 



* Mettenius im Bericht d. 34, Versamml. deutscher Naturforscher, zu Carlsruhe, p. 99. — Von 

 Mohl, Baumfarne {I.e., see p. 291). — Karsten, Veget. Org. d. Palmen, I.e. 



^ Philos. Botan. ed. i, pp. 176, 188. ^ Organographie, I. p. 271. 



* Verm. Schriften, I. p. 184; Physiol. I. p. 443. 



'^ Recherches sur la structure et la fonction des feuilles, Ann. Sci. Nat. i ser. torn. XXI (1830), 

 p. 420, pi. 6-18. 



® [Compare Haberlatidt, Vergl. Anal. d. ass. Gewebesystems, Pringsh. Jahrb. vol. XIII. p. 74.] 



