238 MORPHOLOGY 



gonia seems to be associated with the tendency to differentiate the egg 

 nucleus earlier and earlier in the history of the gametophyte, and when 

 this differentiation occurs before wall formation, archegonia are no 

 longer possible. The compound strobili of Gnetales also, both staminate 

 and ovulate, with their more or less differentiated bractlets investing 

 the stamens and ovules, suggest the inflorescence of certain angiosperms. 

 There is no sure record of Gnetales as fossils, and therefore all the 

 evidence available indicates that they are relatively modern among 

 gymnosperms. Ephedra is evidently related to the Coniferales; and 

 Tumboa and Gnctum are just as evidently related to Ephedra; so that it 

 is altogether probable that the Gnetales represent a somewhat modern 

 offshoot from the Coniferales. 



B. ANGIOSPERMS 



General character. The angiosperms represent the culmination of 

 the plant kingdom, and are plants not only of the highest rank, but also 

 of the greatest importance to man. Probably they constitute also 

 the greatest group of plants in the number of species, which is approxi- 

 mately 125,000. When this vast assemblage of species is contrasted 

 with the 450 living gymnosperms and the 4000 living pteridophytes, 

 it is evident that the angiosperms form by far the largest part of our 

 vascular vegetation. It is also the most modern vascular group, being 

 absolutely unknown in the Paleozoic, and not very abundant until late 

 in the Mesozoic. The conspicuous superficial character of the group, 

 as contrasted with gymnosperms, is implied in the name, the ovule 

 being enclosed by the car pel (megasporophyll) , so that the pollen grain 

 does not reach the ovule, but rests on the surface of the carpel. 



The great groups. The two primary divisions of angiosperms are 

 the Dicotyledons and the Monocotyledons, and the four prominent char- 

 acters used in distinguishing them are as follows: (i) the embryo of a 

 monocotyledon has a single terminal cotyledon and a laterally developed 

 stem tip; white the embryo of a dicotyledon has a terminal stem tip 

 and laterally developed cotyledons (usually two) ; (2) in the stem of a 

 monocotyledon the vascular bundles are scattered; while in a dicoty- 

 ledon they are arranged so as to form a vascular cylinder enclosing a 

 pith; (3) the leaves of monocotyledons have a closed venation, that is, 

 veinlets do not end freely in the margin, which is therefore entire ; 

 while the leaves of dicotyledons have an open venation, veinleis ending 

 freely in the margin, which is often variously toothed or lobed; (4) the 



