PRIMARY PARENCHYMA. LAMINA. 407 



The cells of the chlorophyll-parenchyma are arranged below the epidermis in 

 radial and tangential rows; between them there are always spaces filled with air, 

 usually forming narrow interstices. According to the particular cases, to be 

 exemplified below, their form is rounded- polyhedral, or elongated transversely, i.e. 

 parallel to the surface ; or elongated-prismatic, or cylindrical, and extended vertically 

 to the surface of the entire organ. According to their shape and arrangement the 

 cells of the latter form have been not inappropriately designated palisade-cells, or 

 palisade-parenchyma^ (comp. p. 117). These cells are of approximately equal height 

 in each of the layers parallel to the surface ; between their lateral angles are inter- 

 stices containing air, which either run without interruption along the whole angle; 

 or form those rows of narrow slits described at p. 211. 



The seriate arrangement of the chlorophyll-parenchyma usually becomes less 

 regular at an increased distance from the surface; the palisade-form often passes 

 over into the roundish form. 



According to the form in which chlorophyll-parenchyma, and in certain cases 

 non-equivalent masses of tissue, take part in the composition of the entire organ, two 

 principal modifications of the centric type are distinguished, between which here 

 also intermediate forms are found. 



{a) In many leaves the whole parenchymatous mass is chlorophyll-parenchyma ; 

 towards the middle of the leaf this gradually becomes larger-celled, poorer in 

 chlorophyll, looser, and is often even traversed by large lacimse containing air. To 

 this category belong the leaves, consisting of a few layers of parenchyma, of species 

 of Potamogeton, Ranunculus aquatilis ; the leaves of Chamserops, Copernicia, 

 Klopstockia, Physosiphon, Vanda, Cypripedium spec. ; many Grasses, as Secale, 

 Elymus arenarius, Triticum vulgare ; Yucca filamentosa, with a strongly lacunar 

 central portion; species of Crassula (Fig. 180, p. 378), and Dianthus Caryophyllus. 

 The leaves of the species of Isoetes ^ are also to be mentioned here, with their four 

 air-passages distributed symmetrically over the cross-section, in I. Hystrix and 

 Durieui extending to the epidermis (comp. sect. 51). The leaf of Acorus Calamus, 

 with round-celled chlorophyll-parenchyma in the periphery, and a widely lacunar 

 central portion, may here be mentioned as a transitional form to the second 

 modification. The flat leaves of the first modification described behave in many 

 respects intermediately between the second modification and the bifacial type. 



(3) The entire organ is built up of a many-layered peripheral zone of 

 chlorophyll-parenchyma, and a dissimilar middle portion or middle layer, more or less 

 sharply marked oflf from the former. In the leaf-like stems and halms, all of which, 

 according to what has been said above, belong to this category, this middle portion 

 consists of the ring or cylinder of bundles, to which may be added a surrounding 

 inner zone of large-celled cortical parenchyma, containing little or no chlorophyll 

 (e.g. Salicornia); the chlorophyll-parenchyma forms the peripheral cortical zone; 

 e. g. the halms of Cyperacese and Juncacese, Acorus, Casuarina, Bossisea, INIiihlen- 

 beckia platyclados, Colletia horrida, Cacteae, &c. ; the Equiseta are also to be 

 mentioned here. In numerous foliage-leaves and phyllodes, especially those which 



• Schacht, Lehrb. II. p. 118. 



^ A. Braun, Monatsber. d. Berliner Acad. 1863, P- I54- 



