SECONDARY THICKENING. NORMAL DICOTrLEDONS. ,491 



they seldom exceed the height and breadth stated;— in Cedrus they are as much as 50 

 cells in height, and often more than one cell broad in the middle. In the species 

 first mentioned above, the medullary ray consists firstly of somewhat elongated, pris- 

 matic, procumbent cells, which, on their surfaces of contact with one another and with 

 the tracheides of the ligneous bundles, have, according to the species, one or more large, 

 unbordered pits ; in the latter case they are really not pitted, in so far as the thickening 

 and the pif, strictly speaking, only belongs to the tracheide *. 



Secondly, tracheides occur in addition to these cells of the medullary ray, which they re- 

 semble in form and position. Their walls, where they border on equivalent elements, and 

 on tracheides of the bundles, have bordered pits of smaller size than those of the latter; 

 in many species of Pinus (e. g. P. silvestris, and Laricio), and Sciadopitys, they further 

 have irregular thickening ridges, projecting inwards like teeth, on their upper and lower 

 sides; towards the cells of the medullary ray they only have extremely scattered small 

 pits, which, so far as I could see in P. silvestris, are unbordered. Each of the radial 

 rows of which the medullary ray consists, is, as far as investigation extends, composed 

 exclusively of one of these two kinds of elements, and in fact in a ray more than two 

 elements in height, the upper and lower wedge-shaped edges always consist of one to 

 three series of tracheides. In the middle of the medullary ray there then lie either rows 

 of cells only, or rows of tracheides alternating with the latter. E. g. they occur, in the 

 following order, succeeding one another from above downwards (or conversely), the 

 Roman numerals indicating the rows of tracheides, the others the rows of cells, and the 

 letters (a), {b), &c. the particular medullary rays investigated. 



Pinus silvestris, wood of the stem : («) II, 4, I, i, II. {b) I, 2, 1, 3, I. (<•) I, 3, IV, 3, II. 

 {d) I, 2, I. {e) II, 4, I, &C. 



Larix europcea, stem : {a) I, i, II, 6, I. {b) I, t, IV, 9, I. (c)- I, 14, I, &c. 



Small medullary rays, only two elements in height, are in P. silvestris often composed 

 ■ of 'tracheides only''. 



The, second exception to the usual, purely parenchymatous structure, occurs in 

 several plants which form but little wood, and consists in the fact that the medullary 

 ray is not formed of parenchyma, but of elongated, scXexolic fibrous cells. It has 

 been primarily observed in the perennial stems of the sufFrutescqnt Begonise ", e. g; 

 B. angularis^ muricata, HiigeliL The very large and broad medullary rays of the 

 secondary, wood here consist of upright, very much elongated cells, which abut on 

 one another with oblique, sometimes acute, sometimes blunt, terminal surfaces, very 

 like the cambial cells of the ligneous bundles, and acquire lignified, sclerotic walls of 

 fconsiderable thickness, with small pits. The cells have scanty contents, and fre- 

 quently even contain starch. The broad medullary rays form collectively a tough 

 ling into which the relatively nan-ow ligneous bundles are fitted. 



A similar structure occurs in many herbaceous stems of Umbelliferae, as iri 

 Chserophyllum, Myrrhis, Seseli, Daucus, and Eryngium *, though here more minute in- 

 yestigation is needed, with reference to some doubtful points suggested by Jochinann ; 

 it perhaps occurs frequently in herbaceous Dicotyledons. Of the cases adduced by 



' See Fig. 58, p. 159. Further, Hofmeister, Pflanzenzelle, p. 175. — Sanio, in Pringsheim's 

 jahrb. VIII. . . ■ " .i 



"' ■ "Tor further details see Kraus, Bau d. Nadelholzer,' Wiirzburger Naturwiss. Zeitschr. Bd. V. — . 

 Goppert, Monogr. d. Fossileii Coniferen, Harlem, 1850. Tor figures see especially Goppert, /. c. ; 

 also Schacht, Baum, i Aufl. p. 202 ; Lehrbuch, I. p. 233. 



' Hildebrand,. Begonia-ceen-stamme, p. 24. ... ^ 



* jochmann, UmbelUferarum Structura, p. 10. 



