024 PRIMARY ARRANGEMENT OF TISSUES. 



in -the arrangement of their trachese with the two-limbed bundles of Mono- 

 cotyledons. 



The arrangement of the parenchymatous cells of the xylem results for the most 

 part from that stated in the case of the trachea. In bundles with many rows of 

 tracheae they form similar rows interpolated between the latter, with the form of 

 ■narrow long medullary rays, round which, in a longitudinal direction, the lines of 

 trachea run with slight undulations, alternately receding from one another and coming 

 into contact at the ends of the rows of cells. In bundles of the Monocotyledonous type, 

 with less regular seriate arrangement, they form single longitudinal rows, or groups of 

 various form between the tracheae. The cells themselves are elongated in different 

 degrees, with horizontal or oblique ends, their walls delicate or considerably thickened 

 and lignified ; in the latter case their distinction from tracheides is often difficult. 



(6) The phloem of collateral bundles consists of sieve-tubes, and thin-walled, 

 elongated, prismatic cells, for which Nageli's term Carnbtform-ctWs is to be reserved. 

 With reference to the more special structure three cases, not all equally well known, 

 are to be distinguished. 



1. In the m.ore accurately investigated Monocotyledons (e. g. the Grasses, Fig. 

 150, with which the Equiseta appear also to agree), and no less in very many 

 Dicotyledons, as Ranunculacese, Umbelliferse (Fceniculum, &c.), Vitis, Aristolochia, 

 also Cucurbita, &c., the cross-section of the phloem shows the two kinds of meshes, 

 which have been known since Moldenhawer, and Mohl's Anatomy of Palms ; some 

 wider and polygonal, which are the cross-sections of the sieve-tubes, others narrower, 

 square or rectangular, or frequently narrow and obliquely four-sided, representing the 

 cross-sections of the cambiform-cells. The. latter stand isolated among the sieve- 

 tubes, distributed with varying regularity, so that as a rule each sieve-tube border^ on 

 another with one part of its lateral walls, and with another part on a cambiform-cell. 

 Traced longitudinally the cambiform-cells form rows between the sieve-tubes, and 

 parallel to them. Regarded individually they are as a rule shorter than, seldom as 

 long' as the joints of the sieve-tubes. Both from their arrangement in cross-section, 

 and on tracing them in the longitudinal direction, it often has the appearance as if the 

 cambiform-cells arose with the elements of the sieve-tubes from one mother-cell, the 

 latter dividing longitudinally into a daughter-cell which becomes the sieve-tube element, 

 and another which either becomes a cambiform-cell without further division, or is 

 divided by cross-walls into several of them \ On this point, however, more 

 accurate investigations must be undertaken '^. The cambiform-cells have delicate 

 non-hgnified cell-walls and finely granular protoplasm, with a nucleus elongated in 

 the longitudinal direction. On the structure of the sieve-tubes nothing need here 

 be added to what was said in Chap. V. 



2. In several, perhaps in numerous Dicotyledons (e. g. in the leaf-stalk of Olea 

 Europaea (Fig. 156), in the stem of species of Lobelia, Crassulaceae, Cacteae''; and in 

 several, especially succulent, Euphorbiae, as E. Caput Medusae), -the cross-section 

 of the phloem shows, among wider thin-walled elements, numerous or sparsely 

 scattered groups of much narrower cells, each group often appearing from its size 



^ Compare Vochting, Melastomeen, p. 16. 



^ [Compare \yilhelm, I.e. (see p. 172).] 



" Compare Vochting, Rhipsalideeri, /. r. Tab. 52. 



