]OZ PRTMAR}' ARRAiXGEMENT OF TISSUES. 



examples (e. g. in the Dracaenas). In the one, which may be called the hngitudmally 

 striated, all the bundles run, in the manner indicated, separately from the base to the 

 apex of the leaf or lamina. In the other, with pmiiaie s/riattott, numerous bundles 

 enter the midrib of a flat leaf, and pass through it towards the apex. They then pass 

 one after another from the midrib into one or other half of the leaf, giving off 

 numerous branches into it ; only one or few of them extend to the apex of the leaf 

 itself All the bundles and branches which pass into the halves of the leaf are 

 arranged in a pinnate manner, and have an acroscopically curved direction. The 

 pinnate arrangement is characteristic of the group of Scitaminea?, of the broad- 

 leaved Dracaenas, Curculigo, many species of Hsemanthus (e. g. H. coccineus), 

 Eucharis amazonica, &c. The longitudinally striated arrangement is characteristic 

 of the majority of ordinary, linear, tapered leaves of INIonocotyledons, also for 

 the fan-shaped leaves of Palms, and the Foliola of the pinnate-leaves of the same 

 family. 



The longitudinal bundles which traverse the Monocotyledonous leaves of this category 

 are often of almost equal strength, inasmuch as they enter the leaf from the stem as so 

 many bundles of the trace, or from the node, as almost equivalent branches of one bundle 

 of the trace (e.g. species of Potamogeton). On the other hand, it not unfrequently 

 happens that they are not of uniform origin, some arising as branches from the others, 

 but all pursuing fundamentally the same course. This has already been noticed above 

 for the pinnately striated forms; the same occurs also in the longitudinally striated. 



The longitudinal bundles of one leaf are not uncommonly of almost equal strength; in 

 other cases of very unequal strength. In Palm leaves Mohl distinguishes bundles of three 

 different strengths. Frequently one median bundle, which exceeds the rest in strength, 

 is found in longitudinally striated leaves ; in many leaves of Orchids with five and more 

 projecting ribs (e.g. Stanhopea, Acropera, Maxillaria squalens) there is one bundle in 

 each rib, which is distinguished by its size from the rest, which are not prominent. 

 In the pinnately striated leaves of Heliconia farinosa, the ends of the bundles which 

 pass out from the midrib are much stronger than their branches which pursue a 

 similar course, a difference which cannot be recognised in similar leaves of allied plants, 

 e. g. Phrynium setosum. From the example from the Orchidaceae it cannot be con- 

 cluded that in striated leaves generally the strength of projecting nerves must cor- 

 respond to that of the enclosed bundles. In the keel-like projecting midrib of species of 

 Carex, and in Pandanus pygmaeus, there is a bundle, which far exceeds the rest in strength; 

 in the thick midrib of Zea Mais and other large leaves of grasses there are several with the 

 same arrangement as in the flat halves of the leaf, and resembling those which traverse 

 the latter, with the exception of the rather stronger median bundle. 



The transverse branches which connect the longitudinal bundles like the rungs of 

 a ladder are often almost equal to them in strength, — e.g. Rhapis flabelliformis, Vanda 

 furva, — but usually much weaker, being even reduced to a single vascular tube, or row 

 of tracheides, for instance those in the pinnae of species of Chamsdorea, the leaves of 

 Curculigo, Zea, &c., which are even hard to find. Their number on a given surface 

 varies according to the species: on the average the distance between t\vo may be about 

 1°"", it is often greater, rarely they are much more closely arranged — in Phrynium setosum 

 on the average 10-12 in a distance of i™"". They run either almost exactly transverse to 

 the longitudinal bundles, so that the whole system of bundles consists of rectangular 

 meshes ; or they have a more or less oblique direction. Further, they pass either from 

 one bundle to the next, or in very many cases they pass the next bundle, only touching 

 it externally, and run to the second or third lateral bundle. It is often seen, especially 

 in leaves with alternating stronger and weaker bundles, that they connect those of equal 

 strength, running past the intervening ones of unequal strength. The transverse bundles 



