FILICES 8 1 



in the formation of the pollen of Flowering Plants, occurs sometimes in 

 the two orders just named, and invariably in the Cyatheaceae, Osmun- 

 daceae, and Hymenophyllaceae. Each spore-mother-cell divides at once 

 by cell-walls of cellulose into four compartments, sometimes called 

 ' special mother-cells ' ; within each of these compartments the spores 

 become invested with their permanent cell-walls, both the walls of the 

 original mother-cells and their septa being then absorbed. The spores 

 formed in this way have a rounded cubical shape, while those produced 

 in the mode first described are bilateral, and very commonly kidney- 

 shaped. In the Schizaeacese and Osmundaceae the sporanges are not 

 strictly trichomic in their origin, being formed, before the differentiation 

 of the epiderm, each from a single cell, which may be regarded as the 

 rudiment of a leaf-branch. In both these orders the number of spores 

 produced in a sporange is much larger than in the Polypodiaceae ; in 

 this and in other respects they manifest an approximation to the Marat - 

 tiaceae and Ophioglossaceae. The sporanges of Marattiaceae are alto- 

 gether endogenous in their origin, being developed from an internal 

 mass of tissue, and are destitute of an annulus. On the dehiscence of 

 the normal sporange the spores are at first attached to the annulus, and 

 are detached and thrown to a distance by its sudden return to its 

 original position. The spores of many Polypodiaceae retain their vitality 

 and power of germination for a long period, and require a longer or 

 shorter period of rest before germination ; those of the Hymenophylla- 

 ceae, on the other hand, often begin to germinate while still enclosed in 

 the sporange. In Scolopendrium (Sm.), according to Beck (Verhandl. 

 Zool.-Bot. Ges. Wien, 1879), the exospore does not burst, but decays at 

 the spot where the germinating filament emerges. 



The non-sexual propagation of ferns takes place chiefly by means of 

 the adventitious buds already described, which appear on the lamina or 

 petiole of the leaf (fig. 49). As a normal phenomenon it is, however, con- 

 fined to a small number of species known in cultivation as viviparous or 

 bulbiferous ferns, such as Asplenium bulbiferum (Forst.) and Cystopteris 

 bulbifera (Bernh.). The occasional vegetative propagation of the 

 prothallium has also been already described. 



Ferns are distributed over the whole globe, from the equator to the 

 arctic zone ; and, from the ease of their culture and the beauty of their 

 forms, are favourite objects of cultivation. They are most abundant in 

 moist warm climates, and hence enter largely into the composition of all 

 insular floras. In the tropics a large number of species are epiphytic ; 

 and it is only there, and in the islands of the Southern Hemisphere, that 

 they attain the size of tree-ferns. One or two species are annual, and a 

 single one, Ceratopteris thalictroides, is aquatic, while a very few have a 



G 



