274 



■ PRIMARY Arrangement of tissues. 



V. An'OMalous Monocotyledons. 



Sect; 70. Under this term may be grouped some examples of vascular bundle- 

 systems, which differ fundamentally from that of the very, great majority of Mono- 

 cotyledons. Some of these are found in certain water-plants_. the rest in certain 

 Dioscorese, the bundle-system of which approaches very closely to that of the Dico- 

 tyledons. 



Potamogeton crispus, while it approaches very closely in other anatomical 

 properties to other members of the genus, is distinguished from them by the course 

 of the bundles in the stem. Comp. Figs. 124, 125. 



FIG. 154. — Potamogeton crispus (40). 

 End of a shoot. Longitudinal section 

 parallel to the median planes of the two 

 rows of leaves, cleared by potash. The 

 successive leaves nuinbered r, 2 — 10 ; 

 ■vi, V2 the sheaths of the corre- 

 sponding leaves; the sheaths of the 

 upper leaves were obscured in the pro- 

 cess of preparation, and are partly 

 omitted in the drawing. The median 

 vascular bundles of leaves 9 and 10 are 

 just beginning to develope ; the seven 

 highest leaves are still without bundles. 



Fig. 125. — Potamogeton crispus (40). End of a shoot Thick 

 "median longitudinal section perpendicular to the median plane 

 of both rows of leaves, cleaved by potash; the sheaths were 

 obscured by the process of preparation, and are omitted in the 

 drawing. The successive leaves are numbered in order. The 

 series of unevenly numbered leaves are uppermost, nearest 

 the observer; the even numbers are below. The same is the 

 case with the median bundles, whicH go to the respective series 

 and unite in the axile bundle 7>i, The median bundle is plainly 

 seen as far as the sixth leaf from the apex ; the lateral bundles 

 united to form the two bundles i, I, beginning at the eleventh 

 leaf from the apex (g) in the node; the development of 4 and 5 

 is not yet completed downwards through the intemode. In the 

 intemodes the air-cavities are first formed from below upwards, 

 and from the outside mwards. 



In each of the sheathuig leaves, which alternate in two rows, the bundles pass 

 out at the node. The median bundles run down through the internodes in the 

 manner described for P. lucens and gramineus (Fig, 124). The lateral ones (Fig. 

 -125) pass on each side almost perpendicularly from a bundle which traverses the 

 stem perpendicularly, and corresponds exactly in position -and relatively late* appear- 

 ance to the cauline bundles of the' other species, so that the arrangement of the 



