PHYLOGENY 



often highly ornamental leaves. The microspores develop into pollen 

 (male gametophy tes ) with only three nuclei, one tube nucleus and two 

 sperm nuclei. The macrospores develop into female gametoph^tes w ith 

 only eight nuclei, including the egg nucleus and two polar nuclei which 

 fuse to form the fusion nucleus. The macrospores are completely envel- 

 oped within modified sporophylls, the carpels, which become the fruit 

 when matine. Pollination may be by wind as in the gymnosperms, but it 

 is commonly effected bv insects, occasionally by birds, and rarely by water 

 ( in the case of aquatic plants ) . A unique characteristic of the angiosperms 

 is the phenomenon of double fertilization. As usual, a single sperm nucleus 

 unites with the egg nucleus to form a diploid zygote from which the 

 embryo develops. But in addition to this, the other sperm nucleus unites 

 with the fusion nucleus to form a triploid ( 3 n ) cell from which the bulk 

 of the endosperm is formed. The life cycle of a typical angiosperm is 

 illustrated in Figure 42. 



About a century ago, Darwin called the origin of the flowering plants 

 an "abominable mystery," and there the problem still rests. While bota- 

 nists can agree that the angiosperms must have been derived from some 

 primitive gymnosperm stock, there is nothing but speculation as to which 

 stock may have been the correct one. The fossil record is of little help. 

 The angiosperms suddenly appear in considerable numbers in the Creta- 

 ceous period (late Mesozoic) with no intermediate plants in older rocks. 

 Whatever their origin, the angiosperms quickly became the dominant 

 plants of the world, and they appear to be still on the increase. The class 

 Angiospermae includes two subclasses, the Dicotvledoneae and the Mono- 

 cotyledoneae, whicli are separated mainly on the basis of embr\'ological 

 characters. The monocots include the grasses, lilies, palms, and orchids, 

 while the dicots include the great majority of flowering plants. Anatomi- 

 cally the dicots appear to be the more primitive, and so it is probable that 

 the monocots were derived by reduction of various parts from some primi- 

 tive dicot. 



THE MAIN LINES OF PLANT EVOLUTION 



The main lines of plant evolution may now be summarized witli tlie aid 

 of Figure 43. Whatever the origin of the viruses and the bacteria may 

 have been, the seven phvla of algae must ha\e arisen from bacterial an- 

 cestors, probably from auioug the autotrophic l)acteria. \Vhether the dif- 

 ferent algal phyla arose independently or from a common stock is scarcely 

 indicated by any available evidence. The Cyanophvta or blue-green algae 

 are the earliest plants found iu the h)ssil record; they are extraordinarily 

 primitive in their lack of a distinct uucUmis or jilastids; and thev contain 

 chlorophyll. In view of these facts, it is plausibl(> thai the (;\auoplnta may 

 be ancestral to some other algal phyla, including the Chloroph)ta, yet 

 there is very little evidence that this is actually the case. The origin of 

 the two phyla of molds, Myxomycophvta and Eumycophyta, is also much 

 in doubt. Tluy may have come from bacterial ancestors, or from algae 

 bv loss of chlorophvll, or even from Proto/oa. IIowcncm- much vexed the 



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