. Fundamentals of botany. Botany. 6o8 STRUCTURE AND LIFE HISTORIES much significance should be attached to that fact has been disputed by students of morphology. The gap between the stamen of Cycadeoidea and the type characteristic of modern Angiosperms is partially bridged by the genus Williamsonia (which has simple vs. pinnately compound stamens), and by another genus, Wielandiella, both older genera than Cycadeoidea. From this it has been inferred that the Bennet- titales are a lateral branch, further re- moved than their ancestors from the direct evolutionary stock of the Angiosperms. 522.
. Fundamentals of botany. Botany. 6o8 STRUCTURE AND LIFE HISTORIES much significance should be attached to that fact has been disputed by students of morphology. The gap between the stamen of Cycadeoidea and the type characteristic of modern Angiosperms is partially bridged by the genus Williamsonia (which has simple vs. pinnately compound stamens), and by another genus, Wielandiella, both older genera than Cycadeoidea. From this it has been inferred that the Bennet- titales are a lateral branch, further re- moved than their ancestors from the direct evolutionary stock of the Angiosperms. 522. Origin of Monocotyledons.—If the earliest Angiosperms were dicotyle- dons, as seems to be the case, the mono- cotyledons were probably derived from the dicotyledons by a process of simplifi- cation. Much light has been thrown on this question by a study of the develop- ment of the embryos iembryogeny) of certain plants. The case of Agapanthus umbellatus L'Her. (Fig. 430), a South African plant of the Lily family, may be taken as illustrating the nature of the evidence derived from embryogeny. The sequence of events is as follows.^ As the mas- sive proembryo enlarges, the root-end elongates, thus re- maining narrow and pointed; while the shoot-end widens, becoming relatively broad and flattish. At this broad and fiat end the peripheral cells remain in a state of more active division than do the central cells, and form what is known as the coiyledonary zone. In this zone two 1 The above description closely follows Coulter and Land (1914).. A, monocotyledon- ous embryo; B, dicotyledonous em- bryo. (Redrawn from photo by W. J. G. Land.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Gager, C. Stuart (Charles Stuart), 1872-1943. Philadelphia, P. Blakiston's son & co.
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Keywords: ., bookauthorgag, bookcentury1900, bookdecade1910, booksubjectbotany
