12.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBERS. 55 



111 Aponogeton fenestralis most of the softer tissue decays whilst 

 the leaf is still living, so that the lamina consists of little more 

 than the vascular skeleton. 



The main features of the venation 

 are determined by the type of develop- 

 ment of the lamina. In a pinnate 

 lamina, the venation is pinnate ; in a 

 palmate lamina, palmate ; in a pedate 

 leaf, pedate; in a dichotomously branch- 

 ing lamina, the venation is also dicho- 

 tomous, or as it is specially termed, 

 furcate. But there is considerable 

 variety in the distribution of the smaller 

 vascular bundles : thus the venation of 

 the individual segments of a palmate or 

 a pedate leaf is frequently pinnate. 



FIG. 33. Leaf of a young Fern, 

 with free pinnate venation ; m the 

 midrib ; ss the large lower lateral 

 veins ; n the weak upper lateral 

 veins (x3). 



According to the distribution of the veins 

 and their branches, the following varieties of 



venation may be distinguished ; they are, however, connected by intermediate 

 forms. 



a. The venation is said to be free when the veins end free, without forming 

 anastomoses, at the margin of the leaf ; this is the 



case in the leaves of many Ferns (Fig. 33) ; of Ginkgo 

 (Salisburia), Araucaria imbricata and others, among 

 Coniferae ; of most Cycads ; of Water-Crowfoots, etc. 



b. The venation is said to be parallel, when nu- 

 merous adjacent veins run parallel to each other 

 towards either the apex (Fig. 34) or the margin of the 

 blade, and then unite by curving inwards (Fig. 34 a). 

 They are connected in their course by short veinlets 

 (Fig. 34 v.) which run usually at right angles to them. 

 This form of venation is to be found in the leaves of 

 most Monocotyledons, such as Grasses, Lilies, and 

 Palms, with various modifications. For example in 

 some cases (e.g. Orchis Morio) many veins enter the 

 blade, but they branch scarcely at all ; in other cases 

 lateral veins spring at an acute angle from a midrib 

 which is prominent at the base at least, and then 

 run towards the apex (e.g. Maize and other Grasses, 

 Dracaenas, etc.) ; in others, again, the lateral veins 

 spring almost perpendicularly from the well-developed 

 mid-rib, and run out to the margin parallel to each 

 other, and then turn towards the apex of the leaf 

 (e.g. Canna, Musa, etc.). 



c. The venation is said to be reticulate, when the 



Fie. 34. Apex of a 

 Grass-leaf showing paral- 

 lel venation ; m middle 

 vein; a anastomoses; v 

 veinlots (x4). 



