376 THE ANATOMY OF WOODY PLANTS 



a circular fashion, but occasionally in stems with large leaves 

 provided with numerous foliar traces the periphery of the cylinder 

 is no longer capable of accommodating the bundles, so they have 

 to be disposed of in a position either medullary or cortical. The 

 leaves of the dicotyledons are usually distinguished by skeletal 

 structures or veins ending freely toward the margins. The main 

 veins may be arranged either in a radiating or in a palmate fashion, 

 or may take their origin in an alternating manner from opposite 

 sides of a main vein or midrib, in which case the venation is said 

 to be pinnate. The free venation of dicotyledonous angiosperms 

 gives them a considerable advantage over the monocotyledons 

 in the possibility of submerged existence or a shaded habitat, when 

 their leaves often become finely dissected. The root of dicotyledons 

 presents no special features worthy of note in a general statement. 

 The floral parts are ordinarily present in multiples of five, and 

 the floral envelopes show themselves on the whole less likely to 

 vary from the pentamerous condition than do the essential or 

 strictly reproductive whorls, the stamens and pistils. Pollination 

 is sometimes effected through the agency of currents in the air, 

 but more commonly in the higher families by insects. Fertiliza- 

 tion results from the penetration of the pollen tube from the stigma 

 to the micropyle of the ovule. The course of the pollen tube 

 , after it leaves the region of the style may either be direct through 

 the cavity of the ovary to the micropyle or, avoiding the leap 

 across the ovarial air space, it may make its way round through 

 the basal or chalazal region of the ovule. The last method of 

 fertilization is characteristic of the genus Casuarina, the Betu- 

 laceae, the Juglandaceae, certain Urticaceae, etc., and is known 

 as breech fertilization or chalazogamy. It has been suggested 

 by Treub and Nawaschin that this is a primitive method of pene- 

 tration for the angiosperms and marks a transition from the 

 siphonogamous and higher gymnosperms in which the pollen, 

 being deposited directly upon the ovule, has not become accustomed 

 to leaping an air space. In the lower representatives of the 

 dicotyledons the pollen, although no longer deposited on the 

 micropyle, still maintains its primitive inability to cross an air 

 space. There is much to be said for the hypothesis of the primitive 



