As we pass from Triassic to Rhaetic floras cycadean plants in crease in number and diversity of habit. Our knowledge of these plants is based mainly on external form of the fronds, some sim ple and undivided, others with a lamina dissected into broad, equal or unequal segments (for example, Nilssonia) or, as in the leaves of modern cycads, with separate leaflets attached to a strong rachis (e.g., Zamites, fig. 6, and Otozamites). It has, however, been possible to supplement the distinguishing charac ters afforded by external differences by the more trustworthy cri teria furnished by the microscopical structure of the epidermal layers. We know little of the reproductive organs of many of the Mesozoic Cycadophyta, but in a few instances it has been possi ble to make out the characters of both the male and female or gans. One example may be quoted : Prof. Nathorst described under the name IV ielandiella regularly forking stems bearing in the angles bud-like shoots consisting of a short and thick axis covered with a mosaic of two kinds of appendages, the greater number sterile, slender scales (interseminal scales) with slightly expanded flat tops surrounding similar appendages which were fertile and bore each a small terminal seed. These seed-bearing and sterile scales may be regarded as modified leaves. From the base of the fertile axis "flower" were given off short modified leaves bearing sporangia containing microspores (pollen-grains). The whole fertile shoot was surrounded by linear leaves (bracts). On the forked stem were simple foliage leaves similar in shape to small fronds of a hart's tongue fern but with the blade divided into segments. The plants must have resembled a shrubby Mag nolia, the fertile shoots being superficially comparable to the flower but differing from all existing flowers in the nature of the female organs. lVielandiella agrees in the general plan of its "flowers" with the genera Cycadeoidea and Williamsonia, that came into prominence in the Jurassic period and flourished in the early days of the Cretaceous period. The two latter genera agreed in form and to a large extent in structure with living cycads: Wielandiella on the other hand differed widely both from the two extinct genera and from all recent cycads in the much more slen der and dichotomously branched stem.
Among other genera met with in Rhaetic floras in many parts of the world are Sagenopteris and Podozamites. Sagenopteris is represented by leaves with a few (three to six) narrow oval or elliptical leaflets springing like the fingers of a hand from a single stalk and resembling Glossopteris in the network of veins. The genus has often been compared, on account of leaf resem blance, with the water fern Marsilia, but Dr. Thomas has made out a good case in favour of referring to Sagenopteris two dif ferent kinds of fertile shoots, or catkins, bearing spherical fruits (fig. 8A) or groups of pollen-sacs. The female shoots, described by him as species of Gristhorpia and Caytonia, consist of an axis bearing two rows of short branches each of which carries an apical case containing seeds which is compared with the carpel (pistil) of a flowering plant (fig. 8A-C). These "carpels" differ
from those of modern plants in being modified leaflets, and not formed from complete leaves. The male shoots, referred to the genus Antholithus, are of similar form and their branches bear clusters of four-lobed stamens containing winged pollen-grains. The fruits, first described from the Jurassic rocks of Yorkshire, contain closely packed seeds (fig. 8C) ; on the surface of the wall near the attachment of the fruit to its stalk is a stigmatic lip (fig. 8B) for reception of the pollen. Dr. Thomas instituted a new class, the Caytoniales (from a locality near Scarborough where the fossils were found) for these supposed primitive Angiosperms. Rhaetic and Jurassic floras have long been known to be rich in gymno sperms, particularly in Cycadophyta, but with the exception of a single leaf, which if it had been found in Cretaceous or Tertiary beds would have been referred to a Dicotyledonous plant, no evi dence of the existence of the great class Angiospermae (flowering plants) had been furnished by the earlier Mesozoic floras. One of the attributes of angiosperms is the production of ovules, which after fertilization become seeds, in a closed vessel or ovary, a character implied in the name of the class (seeds contained in a case), in contrast to the naked seeds of conifers and cyads. It is at least certain that the Caytoniales which are represented in Rhaetic as well as in Jurassic rocks, are the most satisfactory examples so far discovered of extinct plants with a definitely angiospermous attribute. The abundance of angiosperms in some of the older Cretaceous floras and the rapidity with which they wandered over the world during the earlier stages of the Cre taceous period suggest that they must have been evolved long before they came to occupy the premier position in the vegetable kingdom. The primitive types of angiosperms probably differed widely in habit from those with which we are now familiar. Sag enopteris may be a pioneer of the present ruling dynasty, which having passed slowly through the earlier stages of evolution reached its full vigour in the early days of the Cretaceous period.
Conifers, though less prominent than Cycadophyta and ferns, were fairly numerous in Rhaetic floras ; some were closely allied to living members of the Araucaria family; others such as Stachyo taxus with its yew-like foliage differed considerably in the fertile shoots from all recent species. Species of Baiera, Ginkgoites and other members of the Ginkgoales with leaves similar in form and in the structure of the epidermal layer to the "living fossil" Ginkgo biloba, are among the most abundant and widely dis tributed Rhaetic plants.