3J4 PRIMARV ARRAXGEMEiXT OF TISSUES. 



nieniioned ; and niiiong the Papilionacese, of which more numerous representatives 

 have been investigated than of other families, a considerable variety of the conditions 

 in question is to be recorded ; as a rule the xylem-plates are diarch in Lupinus varius 

 and Trigonella, triarch in Pisum sativum, Lathyrus sativus, Orobus vernus, Vicia 

 sativa, Ervilia villosa, Ervum Lens, Hedysarum coronarium, Onobrychis sativa, 

 and ]Medicago sativa ; tetrarch in Phaseolus, Dolichos lignosus, and Cicer arietinum ; 

 lastly, higher numbers than fcur occur, as mentioned above, in Vicia Faba. 



In the branches of the root the numbers remain as a rule the same, or diminish if 

 they were greater than two. In subsidiary roots springing from the stem they often 

 increase, in correspondence with the thickness of the roots ; amounting, for example, to 

 7, 9, II in Cucurbita maxima, 5, 6, 8 in Lagenaria and Luffa (van Tieghem), 4-5 in 

 Phaseolus, 5-8 in Valeriana ; the adventitious roots on the rhizome of Nymphasa alba 

 have 6-10 rays; in Nuphar luteum there are as many as 27; in an aerial root of 

 Clusia fiava van Tieghem found 13 rays, and so on. The converse case, however, 

 also occurs ; there is a diarch xylem-plate in all the adventitious and lateral roots 

 of Tropffiolum majus, the main root being tetrarch. 



The orientation of the parts in the cases investigated is such that, in the case of 

 diarch and tetrarch structure of the inain root, the surface or one of the two inter- 

 secting surfaces of the xylem-plates always coincides with the median plane of the 

 two cotyledons, which diverge at an angle of 180°. In the triarch main roots of 

 Pisum, and the other triarch Papilionaceae mentioned, the planes of two xylem-plates 

 fall according to van Tieghem in the median planes of the two cotyledons, which 

 only diverge at an angle of 120°. For higher numbers exact statements are wanting. 

 In all Phanerogams the ])lane of the diarch xylem-plates of lateral roots always lies 

 in the median plane of the main axis from which they arise, and the same applies, so 

 far as investigated, to one of the planes in tetrarch xylems. 



The original structure of the individual bands of tissue shows — within the 

 general plan of structure of root-bundles — few peculiarities characteristic of Dicoty- 

 ledons. As regards the xylem-plates the usually very gradual increase of the width 

 of the vessels in the centripetal direction is worthy of remark. Only as an exception, 

 in the polyarch subsidiary roots on the rhizome of Primula Auricula and Nym- 

 phseaceae does the case usual in Monocotyledons occur, namely, that the short row 

 of vessels, which does not reach to the centre, consists of a few narrow peripheral 

 vessels, and then of one or several w-hich are very w^ide (Fig. 164). In most cases 

 belonging to this series the one- or few-rowed plates constitute radial bands, narrow 

 in cross-section, separated by relatively very broad interstices. These bands either 

 meet in the middle, or they are separated, or connected together, by means of a 

 parenchymatous axial strand. In stout polyarch subsidiary roots, and in the upper 

 part of stout main-roots, where they pass over into the hypocotyledonary stem, this 

 axial parenchymatous mass, the 'pith' of the root, is of considerable thickness. 

 Rarel}', among Dicotyledons, the axial parenchyma connecting the xylem-rays is 

 represented by a strand of sclerenchymatous fibres, e. g. in the subsidiary roots of 

 Stachys sylvadca, ^Mentha aquatica, and Hedera Helix (v. Tieghem). 



A peculiarity, which so far as I am aware only occurs among Dicotyledons, is the 

 presence of a bundle of sclerenchymatous fibres, roughly crescent-shaped as seen in 

 cross-section, on the outside of the phloem-groups of triarch and tetrarch roots of 



