PHANEROGAMS. 



421 



layers, which are arranged in vertical laminae disposed in radiating planes separated by tracts 

 of muriform parenchyma ; successive additions are made to the outer margins of the woody wedges 

 previously formed through the agency of a pseudo-cambial layer of the innermost ' bark.' These 

 exogenous growths continued until the woody zone attained to a great thickness in the larger 

 trunks. These exogenous layers took no part in supplying the leaves with vessels. The foliar 

 bundles invariably pass through them on their way from their source in the inner non-radiated vas- 

 cular cylinder to the leaves. It being now admitted that Stigmaria was the general form of root 

 of Lepidodendroid and Sigillarian types it is necessary to correlate its tissues with those of the 

 aerial stem. It contains a 'medulla' surrounded by a cylinder composed of radiating vascular 

 laminae separated by cellular rays, and enclosed in a thick ' bark.' Large vascular bundles are 

 given off from the vascular wedges to supply the rootlets. Thus the structure of the root differs 

 from that of the aerial stem in two ^vays. (i) The inner vascular cylinder of the latter, character- 

 ised by the non-radiating arrangement of its vessels, by the absence of ' cellular rays,' and by the 

 numerous foliar bundles which it gives off to the leaves, is altogether wanting in the former. On 

 the other hand, the exogenous zone of the stem is prolonged into the roots, retaining all its more 

 important features. These however are modified in two ways — ist, in the absence of small passages 

 for the transmission of foliar bundles of vessels ; and, 2nd, in their replacement by much larger 

 spaces having a lenticular section, and through which large vascular bundles, directly derived by 

 enlarging from the exogenous laminre themselves, pass outwards to the succulent rootlets. That 

 Lepidostrobi are the fruits of Lepidodendroid plants is certain. Equally so is it that many of the 

 former produced microspores in the upper sporangia of each cone, and macrospores in those oc- 

 cupying its basal end. The incalculable myriads of these macrospores found in many coals renders 

 it probable that a very large number of the Lepidostrobi possessed both kinds of spores ; indeed 

 it is far from certain that any of them did otherwise. In the great majority of cases the spor- 

 angia of these fniits are shrivelled and empty, the spores having been shed ; and this renders it 

 impossible to say what their original character was*.' — Ed.] 



GROUP V. 

 PHANEROGAMS. 



The Alternation of Generations in Phanerogams is concealed in the formation 

 of the Seed, which, at least in its earliest stage, consists of three parts:— (i) The 

 Testa, which is a part of the mother-plant; (2) The Endosperm'^; and (3) The 

 E?nbfyo, the result of the development of the oospore or fertilised embryonic vesicle 

 (oosphere). 



1 [For the literature of the Carboniferous Lycopodiacese see Brongniart, Archives du Mus. 

 d'Hist. Nat. vol. I, and Journ. Bot. vol. VII, pp. 3-8.— King, Edin. New. Phil. Journ. vol. XXXVI. 

 —Hooker, Mem. Geol. Surv. vol. II.— Carruthers, Monthly Mic. Journ. vol.1, pp. 177-181 and 

 225-227 ; Quart. Journ. Geol. Soc. vol. XXV, pp. 248-254.— Williamson, Phil. Trans, vol. CLXII, 

 pp. 197-240.— Thiselton Dyer, Quart. Journ. Mic. Sc. 1873, pp. 152-156.— Ed.] 



2 The only reason why the ripe seeds of many Dicotyledons do not contain any endosperm is 

 because it has already been absorbed and supplanted by the rapidly growing embryo before the seeds 

 become ripe ; while in others this absorption happens only on germination after the ripenmg of 

 the seeds, i. e. on the unfolding of the embryo ; more rarely the formation of endosperm is from 

 the first rudimentary. 



