FILICINEAE. HOMOSPOROUS FILICINEAE. 



207 



as a rule produces no archegonium ; when the prothallium has grown into the 

 cordate form, a prominence appears on its under side not far from the growing point, 

 and this prominence developes into a leaf. The apex of the stem is formed near the 

 base of the leaf and soon gives rise to a second leaf, and as it increases in size 

 assumes the typical character of the fully-developed state. The primary root is 

 formed inside the tissue near the insertion of the primary leaf; the point of origin of 

 this primary root is generally in the base of the leaf, but it may be in the prothallium, 

 if the vascular bundle is prolonged so far. The succeeding roots are formed in the 



FIG. 154. Adiantitm Capillus Veneris. Longitudinal section 

 through the prothallium // and the young Fern E ; h root-hairs, 

 a archegonia of the prothallium, * the first leaf, tu the first root of 

 the embryo. Magn. about 10 times. 



FIG. 155. Adiantum Capillus Veneris. The prothallium pp seen from below has a young fern-plant attached to 

 it ; * the first leaf, -a/ and ia" the first and second root, /* root-hairs of the prothallium. Magn. about 30 times. 



same way as in the plant produced from an oospore. Thus the formation of the 

 organs in the case of the young plant which is produced as a shoot from the pro- 

 thallium is similar to their formation in the plant that springs from an embryo, in- 

 asmuch as the origin of the organs first formed (the root and leaf) is independent of 

 the primary bud, whereas all succeeding leaves are outgrowths from it. Two leaves 

 also may be formed instead of a single primary leaf, and two primary roots, or two 

 stems by the side of the primary leaf; such occurrences show that these organs arise 

 independently of one another. There are other abnormalities connected with this 

 interesting point, which cannot be mentioned here. Antheridia often in numbers 

 are found on these apogamous prothallia of Pteris cretica ; but in much the greater 

 number of them there is no trace of an archegonium ; of some hundreds examined by 

 De Bary only seven had one archegonium each, and none of these seven appeared 

 capable of fertilisation. In Todea africana^ on the contrary, according to Sadebeck 

 archegonia are almost invariably formed on the apogamous prothallia, and De Bary 

 found them comparatively abundant in Aspidium falcatum, but these archegonia, 

 though apparently of normal structure, never showed signs of fertilisation, but died off, 

 while young ferns were being produced asexually as shoots from the prothallium. 

 Lastly, no archegonia occur on the apogamous prothallia of Aspidium Filix-mas var. 

 cristatum. This plant at the same time, as De Bary has pointed out, suggests an ex- 

 planation of the origin of apogamy ; it is a comparatively new garden variety of the 

 common Aspidium Filix-mas. The prothallia of the latter have sexual organs with 

 the usual functional power and produce normal embryos. Sexuality, it would seem, 

 has disappeared in the variety cristatum, and been replaced by an asexual formation 

 of shoots, and the same may be inferred in the case of all other apogamous Ferns. 



