41 a PRIMA Ry ARRANGEMENT OF TISSUES. 



side of the lamina of the leaf of Chamaerops humilis it is thick, large-celled and 

 many-layered over the main ribs, single-layered and small-celled elsewhere. 



Sect. 122. The primary cortical mass of the roo/^ consists as a rule ex- 

 clusively or chiefly of parenchyma, the cells of which, corresponding to the original 

 arrangement of the meristem, remain arranged in vertical longitudinal rows. (Com- 

 pare pp. 9-13.) 



In the roots of Phanerogams the cortex is more or less sharply severed into two 

 layers, an outer and an inner. The former consists of cells which are usually, 

 though not always, narrower, and are uninterruptedly, or at least very closely, united ; 

 in the thicker roots they show, as seen in cross-section, a polyhedral form, are 

 arranged in several concentric, but not in accurately radial rows, and not unfrequently 

 have somewhat thick, collenchymatous walls, which often become sclerotic. In 

 thin roots, e.g. Hordeum, Elodea, Lemna, &c. it is a single hypodermal layer of 

 cells. The inner layer consists in very thin roots, as in those of Lemna minor, 

 of two concentric strata of cells as a minimum, usually of several such strata, while 

 in thick roots they are very numerous. (Fig. 168, p. 360.) The innermost of them 

 is always the endodermis, surrounding the axial bundle. Further, as often mentioned 

 in former paragraphs, the cells of the successive strata are as a rule permanently 

 arranged in radial rows, which are very regular, especially in Monocotyledons. 

 Between their rounded corners they leave spaces containing air, which have the 

 greatest average width in the middle portion of the layer. The larger lacunae and 

 passages containing air which occur in roots (comp. Sect. 51) belong to the inner 

 layer, or in thick roots to its middle portion. The fibrous thickenings on the walls, 

 described at p. 117, as occurring in Orchidea^ and Coniferae, always belong prin- 

 cipally or exclusively to the inner parenchymatous layer of the root. 



The successive concentric strata of cells of the inner layer arise, with few 

 exceptions, from the initial cells by tangential divisions in centripetal order, in Acorus 

 Calamus and other Aroids centrifugally ; in several cases (Zea, Helianthus, Palms) 

 the later tangential divisions appear in less regular succession. Where the whole 

 cortex proceeds from a single initial layer, the first tangential division of the latter 

 (leaving the epidermis out of consideration) seems always to cut off" the outer layer, 

 which then either remains undivided, or undergoes a few further tangential divisions, 

 which in Stratiotes take place in strictly centrifugal succession. In Linum (p. 12) 

 a special initial layer for the single external layer of the cortex extends round the 

 meristematic apex. In the type of root described at p. 12, with a transverse 

 common initial zone at the growing-point, the succession of the tangential divisions 

 likewise appears not to be quite regular. 



The single layer of cells bordering on the epidermis, which, as already stated above, is 

 often distinguished by a special structure, and always by uninterrupted lateral connection 

 — ^of its cells, is named endodermis by Nicolai. As in the present work this word has been 

 used for layers of cells which, without reference to the place of their occurrence, are 

 characterised by a definite structure, this term cannot be maintained. According to its 

 position and origin, this layer, which in agreement with the terminology here constantly 



* Compare the works of Nicolai, Nageli and Leitgeb, Van Tieghem, Janczewski, &c., which 

 are cited at pp. 7, 351, 356. — Reinke, in Hanstein's Botan. Abhandl., Heft 3. [Also, Olivier, Re- 

 cherches sur Tappareil tegumentaire dcs Racincs. Ann. Sci. Nat. Ser. 6, Tom. 11, 1S81.] 



