• 360 



PRIMARY ARRANGEMENT OF TISSUES. 



different in certain zones and groups. Of the combinations which are here possible 

 the following usually occur: (i) the entire interstitial mass of cells, including the 

 cylindrical axial portion, remains thin-walled and parenchymatous, e. g. adventitious 

 i-oots of Tradescantia virginiana, Curcuma longa, and Clivia nobilis (Acorus Calamus is 

 intermediate, i. e. its parenchyma has very strong walls). (2) The whole mass of cells 

 mentioned becomes sclerenchymatous, e. g. Garex divulsa, Cyperus alternifolius, and 

 no doubt most adventitious roots of Cyperaceae and Grasses; also Curculigo- recur- 

 Vata. (3) The tracts of cells between the xylem- and phloem-plates are sclerenchy- 

 matous, forming together with them a dense firm ring around an axial strand of 

 parenchyma, with intercellular spaces containing air : e. g. roots of Smilax (Sarsa- 

 parilla) with very extensive parenchyma containing abundance of starch, most aerial 

 roots of Orchidaceas (e. g. Epidendron ciliare, Oncidium sphegiferum), and many 

 Palm roots (cf Mohl, /. c), in which scattered sclerenchymatous fibres may occur 

 again inside the axial thin-walled parenchyma, e. g. Chamsedorea elegans. 



The pericambium remains in most cases thin-walled and parenchymatous, even 



where it borders on sclerenchyma, 

 though it may itself eventually be 

 involved in the sclerosis, either wholly 

 or in part ; the former, for example, 

 is the case in Sarsaparilla roots, the 

 latter among Orchids, e. g. Epiden- 

 dron ciliare, where opposite each 

 xylem-plate two rows of its cells usu- 

 ally remain very delicate, while the 

 others, like those adjoining them on 

 the inside, become greatly thickened. 

 The usually one-sided sclerosis of the 

 endodermis, which is very frequent, 

 though by no means universally dis- 

 tributed among long-lived Mono- 

 cotyledonous roots, has been dis- 

 cussed in Sect. 27. 



Deviations from the type of 

 structure of Monocotyledonous roots 

 hitherto considered occur in difr 

 ferent degrees as regards the ar- 

 rangement of the forms and regions of tissue, though their structural conditions 

 remain the same. 



In thick root-bundles, some or all of whose xylem-plates converge in pairs in 

 the form of a V, the groups of phloem lying inside a V are often smaller than those 

 between two Vs. The latter are frequently large, radially-placed plates, while the 

 former are roundish groups. This occurs in an exquisite form in the aerial roots of 

 an Aroid cultivated in Halle under the name of Philodendron Imbe (Fig. 168), and 

 also in Palms; cf. Mohl's figure of Diplothemium maritimum already cited. In 

 Chamsedorea elegans this inequality goes further. In the corner of the V lies a 

 small roundish group of phloem ; between every two V's a similar group lies towards 



Fig. 168. — Philodendron Imbe Hort. Halens. Cross-section 

 tliroiigh a thick subsidiary root, slightly magnified. The axial 

 vascular bundle, and on the right the entire cortex, are shown ; g 

 external limit of the xylem-rows. The obliquely shaded radial 

 bands -w are the phloem-groups; / periderm; b fibrous bundle 

 surrounding an intercellular passage containing latex. 



