The class of _one seed-lobed plants_ (Monocotylæ, or Monocotyledons, also called Endogenæ) comprises those flowering plants whose seeds possess but one germ leaf or seed lobe (cotyledon). Each whorl of its flower contains in most cases _three_ leaves, and it is very probable that the mother plants of all Monocotyledons possessed a regular triple blossom. The leaves are mostly simple, and traversed by simple, straight bunches of vessels or “nerves.” To this class belong the extensive families of the rushes, grasses, lilies, irids, and orchids, further a number of indigenous aquatic plants, the water-onions, sea grasses, etc., and finally the splendid and highly developed families of the Aroideæ and Pandaneæ, the bananas and palms. On the whole, the class of Monocotyledons—in spite of the great variety of forms which it developed, both in the tertiary and the present period—is much more simply organized than the class of the Dicotyledons, and its history of development also offers much less of interest. As their fossil remains are for the most part difficult to recognize, it still remains at present an open question in which of the three great secondary periods—the Trias, Jura, or chalk period—the Monocotyledons originated. At all events they existed in the chalk period as surely as did the Dicotyledons. [Illustration: _Haeckel History of Creation._ _PL. V._ Relative lengths of the 5 Epochs in percentages. Quarternary Epoch 0.5 Tertiary Epoch 2.3 Secondary Epoch 11.5 Primary Epoch 32.1 Primordial Epoch 53.6 ----- _Total_ 100.0 Single-stemmed or MONOPHYLETIC PEDIGREE of the VEGETABLE KINGDOM based on Palæontology.] The second class of plants with enclosed seeds, the _two seed-lobed_ (Dicotylæ, or Dicotyledons, also called Exogenæ) presents much greater historical and anatomical interest in the development of its subordinate groups. The flowering plants of this class generally possess, as their name indicates, two seed lobes or germ leaves (cotyledons). The number of leaves composing its blossom is generally not three, as in most Monocotyledons, but four, five, or a multiple of those numbers. Their leaves, moreover, are generally more highly differentiated and more composite than those of the Monocotyledons; they are traversed by crooked, branching bunches of vessels or “veins.” To this class belong most of the leafed trees, and as they predominate in the tertiary period as well as, at present, over the Gymnosperms and Ferns, we may call the cænolithic period that of leafed forests.