200 THIRD GROUP. VASCULAR CRYPTOGAMS. 



appearance. A few of these prothallia are dichotomously branched, the mid- rib 

 bifurcating and each of the secondary mid-ribs running into one of the two branches 

 of the prothalliu.m. It is highly probable that this branching is brought about in the 

 same way as in the thalloid Hepaticae, in Pellia fo instance, and a branching which 

 agrees apparently with that of Metzgeria has been noticed in Hemitelia gigantea *. 



Gymnogramme leptophylla 2 may be adduced here as an example of a species of 

 Polypodiaceae which departs somewhat more widely from the typical forms. The 

 germination of the spore produces at once the rudi ent of a spathulate prothallium. 

 But its apex does not become the apex of a prothallium which is to be ultimately 

 heart-shaped, but lateral outgrowths are formed on both sides of the prothallium or 

 only on one ; these lobes again form new lateral branches, and the result is a many- 

 lobed prothallium. The archegonia do not arise as in the rest of the Polypodiaceae 

 on a cushion-like mass of tissue ; instead of such a cushion there appears, first of all, 

 a conical outgrowth from the prothallium, which thrusts itself into the ground and 

 there soon puts on the appearance of a small tuber, gradually filling its inner cells 

 with reserve-material, oil, &c., and losing at the same time its green colour. On this 

 structure which is known as the fertile shoot, and on its upper flattened side which is 

 towards the prothallium, the archegonia are produced, and if their oospheres are not 

 fertilised, two new prothallium-lobes grow out from the tuber. Usually the prothallia 

 die away when a fertile shoot is produced, but they are perpetuated by adventitious 

 shoots, which spring either from their margin or from their surface. These flat 

 adventitious shoots often take the form of small tubers that are like the fertile shoot, 

 but are distinguished from it by originating in any part of the surface of the thallus 

 and by never producing archegonia, but only antheridia, as will be readily understood 

 from what has been said above respecting the causes which lead to such variations. 

 The tubers may lie dormant for a time and especially withstand desiccation and other 

 changes, and eventually give rise to a new prothallium ; and while the prothallium, 

 the sexual generation, of Gymnogramme leptophylla is thus perennial, the asexual 

 generation dies down after forming its spores, and is therefore annual. Marginal 

 adventitious shoots occur also in the prothallia of others of the Polypodiaceae, as in 

 Aspidium Filix-mas, Notochlaena, &c. ; also in Osmunda, Ceratopteris and other genera. 

 Peculiar gemmae formed of rows of cells have been recently described by Cramer 3 

 from the prothallia of Ferns, which belong probably to the Hymenophylleae, but may 

 be only prothallia of other Ferns altered by disease. 



The development of the prothallium in the Cyatheaceae 4 differs from that in the 

 Polypodiaceae, as already described, only in subordinate points, and the development 

 of the same organ in the Gleicheniaceae 5 agrees in the main with that of the typical 

 prothallia of the Ferns ; but like the Osmundaceae they sometimes develope a cell- 

 mass at once instead of a cell-surface. The germination of the Hymenophylleae 6 is 



1 Bauke in Beilage zur Bot. Ztg. 1879, Taf. 5 u. 6. 



2 Goebel, Entwicklungsgesch. d. Prothalliums von Gymnogramme leptophylla Desv. (Bot. 

 Ztg. 1877). 



8 Ueber d. geschlechtslose Vermehrung d. Farnprothalliums (Denkschr. d. Schweiz.-naturf. Ges. 

 Bd. XXVIII, 1880). 



* Bauke in Pringsheim's Jahrb. Bd. X. 



5 Rauwenhoff in Bot. Zeit. 1879, P- 44 J - 



6 Mettenius, Ueber d. Hymenophylleen (Abh. d. K. Sachs. Ges. d. Wiss. VII Bd.). Janczewski 



