300 



PRIMARF ARRANGEMENT OF TISSUES. 



lie internally : internal endings. The bundles which end at the margin and point may 

 be termed, in accordance with the terminology of the coarser nervation, apically 

 directed (aerodrome) and marginally directed (craspedodrome). The points of end- 

 ing of strong marginally directed bundles are often the ends of teeth and laciniae. 

 Anastomoses may appear between branches of any order, between equivalent and 

 non-equivalent ones, and at any point of the foliar expansion. They give the bundle 

 system the form of a net (reticulate veins), which varies greatly in individual 

 instances. An allied special form, which is especially common in flat leaves, is 

 found where anastomosing bundles describe curves close within the margin and 

 following its outline : these are curved (camptodrome) bundles, according to the 

 terminology of nervation. 



The extremely various individual cases, which arise by various combinations of 

 the above phenomena, group themselves under two main types, namely expansions 

 with bundles having a separate course, which end free, without anastomoses ; and 

 such as have anastomosing bundles. 



I. Bundles ivith separate course and free ends are found in the rudimentary and 

 submerged leaves of many Angiosperms of the most various orders, in the foliar 

 expansions of all Gymnosperms, with the exception of Gnetum and Stangeria, and in 

 the leaves of many Ferns. One unbranched bundle, or the ramifications of branched 

 ones, traverse the foliar expansion, and end free either internally or usually at the 



margm. 



Rudimentary scale-leaves of Angiosperms often have their vascular bundles thus 

 arranged, when they are present at all ; the same may be said of the cotyledons of Mo- 

 nocotyledonous plants with one median bundle, or with two running near to the middle 

 line, or with more than two. The cotyledons of Dicotyledonous plants have been but 

 little investigated with regard to the relations in question, many have certainly a reti- 

 culum of bundles, even when they are ' single-nerved.' These most simple forms 

 of leaves have been but little attended to in relation to the structural conditions under 

 consideration. Of larger foliar expansions many submerged leaves of Dicotyledons 

 (Batrachium, Myriophyllum^) with one bundle in each segment of a leaf belong to 

 this series; also Pseudocallitriche with one median bundle^; Elatine Alsinastrum 

 with one median bundle, which usually gives off some marginally directed branches 

 into the narrow submerged leaves, &c. ; similarly among Monocotyledons, e. g. the 

 rudimentary simple median bundles of the Hydrilleae. In the foliage of land-plants 

 there is one simple apically directed bundle in each of the scale-like rudimentary 

 leaves of the Casuarinas and of Arceuthobium, as also in those of Equisetum and 

 Ephedra, which resemble them in habit. 



Among Gymnosperms the foliar expansions of all Conifers ^ belong to this 

 category : the leaves of the Cupressineae, of Taxus, Phyllocladus, &c., with one 

 median bundle ; those of the Abietinese which usually have two very close together, 

 with median parallel course, rarely (Abies Pindrow) with one simple bundle ; the 

 double leaves of Sciadopitys with two which have a parallel course near the median 



* Askenasy, Botan. Zeitg. 1870, p. 19^. — Vochting, Myriophyllum, I.e. 



* Hegelmaier, Monogr. d. Gattg. Callitriche, p. 31. 



' Geyler, I.e. (see above, p. 245). — ThonuT.§, in Fringsheim's Jalirb. IV. p. 43.— Stiasburger, I.e. 



