39« 



VASCULAR CRYPTOGAMS. 



be understood in the same manner, A uniformly tliick envelope is formed round 

 the spore with the exception of the apex (Fig. 299 ex), consisting of distinct prisms 

 arranged radially, the walls of which are, however, much denser, so that the whole 

 layer gives the impression of a honeycomb ; but the prisms are not hollow, like 



empty honeycomb-cells, but are filled with a 

 less dense substance. A hyaline gelatinous 

 envelope is also present in the spores of 

 Marsilea, leaving the apex free, but is con- 

 siderably elevated around it, and thus forms 

 the deep funnel ; the concentric stratification 

 can be seen in it with peculiar clearness (Fig. 

 286 si). I cannot doubt that the structure 

 of the thick exospore of Salvinia is also only 

 the result of more complicated differences in 

 density. The microspores so far resemble 

 the macrospores that their exospore has also 

 a firm inner layer which is cuticularised and 

 shows an internal structure depending on dif- 

 ference of density. This layer is surrounded 

 in Marsilea by a thick, in Pilularia by a 

 thin hyaline envelope capable of swellmg in 

 water. 



The Class Rhizocarpeae contains, besides 

 the genera that have already been mentioned, 

 only one other, Azolla, which, although not 

 yet accurately known \ is nearly allied to Sal- 

 vinia. The four genera form, therefore, two 

 groups ; Sah'miece (Salvinia and Azolla), and 

 MarsilecB (Marsilea and Pilularia) ; the further 

 characteristics of which need not be further 

 alluded to. 



The Formation of the Tissue of Rhizocarps 

 is simple in comparison to their external 

 differentiation. An axial fibro-vascular bundle 

 passes through th^ root, stem, and leaf-stalk, 

 and in the lamina of Marsilea dichotomises 

 many times and anastomoses at the margin. 

 As is usual in water and marsh plants, the 



Fig. 299. — Longitudinal section through the spore 

 prothallium and embryo o^ Marsilea salratrix (y.a.ho\.\t 

 60); am starch- grains of the spore, i inner coat 

 of the spore burst above into lobes, ex the exospore 

 consisting of prisms, c the cavity beneath the arclied 

 diaphragm on which is the basal layer of the pro- 

 thallium, ft the prothallium, wh its root-hairs, a the 

 archegonium.y the foot of the embryo, 7u its root, s 

 the apex of its stem, b its first leaf by which the pro- 

 thallium becomes extended, si the mucilaginous enve- 

 lope of the spores which at first forms the funnel above 

 the papilla, and which still envelopes the prothallium 

 50 hours after the dissemination of the spores. 



parenchymatous fundamental tissue contains 

 large air-canals placed in a circle as seen in transverse section and separated by radial 

 lamellce of tissue one cell thick. In Salvinia the parenchyma is composed everywhere 

 of lamellae of tissue (Fig. 294 D) w^hich bound the capacious air-cavities that lie one 

 over another in the aerial leaf like cells of a honeycomb. The outer layer of cells is 

 differentiated into an epidermis with hairs and stomata on the leaves and walls of the 

 sporocarps ; the stomata are small and of very peculiar form ^. 



^ [See Strasburger, Ueber Azolla : Jena 1873. — Ed.] 



2 On the peculiar interstitial stride of the leaves of Marsilea see Braun (Z. c, p. 672) ; on the 

 histology of the sporocarp see Russow, /. c 



