STRUCTURE OF RADIAL BUNDLES. 359. 



become wider quite gradually in centripetal order, e. g. in many Orchids, as Stan- 

 hopea sp., Epidendron ciliare, &c. Here also the number of the vessels of a plate 

 following one another in the radial direction is small, on the average 4-6, not un- 

 frequently still fewer. In the Carices investigated the row usually consists of a single 

 narrow peripheral vessel, or two lying side by side in the tangential direction, and 

 one wide internal pitted vessel, the latter being separated from the former by at least 

 three layers of parenchymatous cells. A second narrow pitted vessel may lie between 

 the two. The peripheral vessels often occur without the corresponding wide one, so 

 that a row can no longer be spoken of. 



In smaller roots or bundles, as in the main roots mentioned above, the weaker 

 roots of all degrees among the Grasses (Secale, Triticum), and in weak adventitious 

 roots of Tradescantia virginica, the xylem-rows either meet in the middle of the bundle, 

 or converge towards one or two wide vessels passing through the centre of the bundle, 

 which originate very early, but attain their development very late. The xylem-rows 

 sometimes come into direct contact with these, or are sometimes separated from 

 them by a few interstitial cells. In the thicker typical roots of Monocotyledons the 

 radial xylem-plates do not nearly reach the centre. The latter is occupied by a 

 thick cylinder of parenchyma or sclerenchyma, at the circumference of which the 

 system of xylem-plates often forms a relatively narrow ring. 



In the great majority of roots of Monocotyledons the xylem-ring is surrounded 

 on the outside by an uninterrupted peric&nbiUm, which is one layer thick over the 

 xylem-plates, and the outside of which borders on the endodermis. It rarely con- 

 sists of two layers over the xylem-plates, as in the main roots of Tulipa Gesneriana 

 mentioned above, and in roots of Sarsaparilla. All the roots of Graminese investi- 

 gated form a remarkable exception to this rule (Oryza ^, Secale, Triticum, Zea, Coix, 

 Sorghum, Hordeum, and Paspalum spec. ''), as in these the pericambium is, as a 

 rule, interrupted by the rows of vessels which thus border directly on the endodermis. 

 Even here, however, a small pericambial cell often lies between the endodermis and 

 the outermost vessels, e. g. in Maize. The same occurs among the Cyperacese in 

 species of Carex. In certain cases the narrow pitted vessel borders closely on the 

 endodermis, e. g. in C. foenea, foUiculata, divulsa, and hirta ; or both this and the usual 

 arrangement, in which a pericambial cell is present outside the vessel, may occur in 

 different parts of one and the same cross-section. According to van Tieghem, 

 other species of Carex, such as C. brizoides, only show the latter typical arrange- 

 ment ; the same is the case in species of Cyperus, as C. longus and C. alternifolius. 



In these typical roots the structure of the single tissue-elements shows a variety 

 of individual differences, both as regards the vessels, and no doubt the sieve-tubes 

 also, though the latter in most cases still require more accurate investigation. Into 

 these differences we cannot here enter at all minutely. The mass of cells, forming 

 simultaneously longitudinal and concentric rows between and inside the xylem-plates, 

 shows sometimes a typically parenchymatous, sometimes a typically sclerenchymatous 

 structure, or a form intermediate between the two. And indeed these peculiarities 

 either extend uniformly to the whole interstitial mass of tissue in question, or are 



' Nageli und Leitgeb, /. c. 



' Van Tieghem,. /.f. [See further, Klinge, Vergl. hist. Unters. d. Gramineen u. Cyperaceen- 

 wurzeln, Mem. de I'Acad. Imp. St. Pelersb. VII. Ser. Tom. XXVI. No. I3, 1879.] 



