12 



SPORES AND THALLIDIA. 



Mosses, and belonging for the most part to tropical regions — the veins of the pinnw 

 project beyond the margin of the green tissue and form styloid processes whose 

 epidermal cells become the points of origin of sporangia. Each styloid process 

 thus constitutes an axis bearing the sporangia, and the entire sorus has the form ol 

 a little spike. But the sorus itself stands in a cup formed by an upgrowth of the 

 green tissue at the margin of the pinna (see figs. 189 ^ and 189 ^). 



In the three groups of Ferns above dealt with the sporangia arise from epidermal 

 cells. In the Gleicheniacese and Schizseacese (two specimens of which are shown in 

 figs. 189^ and 189^) the sporangia are modified leaflets. We must here remark that 

 the fronds of Ferns in spite of their similarity to foliage-leaves are not to be regarded 

 as such, but as phylloclades, whilst the scales upon the fronds must be considered 

 to be leaves. We shall refer to this again later on. Now, in Gleicheniacese and 

 Schizseace^ some of these small scaly leaves are metamorphosed into sporangia 

 which here take the form of rounded bodies set in rows of pit-like cavities 

 hollowed out of the pinnae, whilst other scales constitute protective coverings to 

 these sporangia. The relation existing between the various parts in the case of a 

 pinna of Gleichenia alpina is shown very clearly on an enlarged scale in fig. 189 '. 



In respect of origin and development the spores and sporangia are again quite 

 diflPerent in the group of Ferns comprised under the name of Ophioglossese, one 

 species of which — viz. the spear-shaped Moonwort (Botrychium lanceolatum) — is 

 represented in fig. 189 ^. In these Ferns, the sporogenous portions take the form of 

 nests of cells embedded in the tissue of the frond. The cells in these niduses 

 become partitioned each into four chambers, and the latter contain protoplasts, 

 which surround themselves with membranes and become spores. The spores are 

 set at liberty as a consequence of the solution of the walls of the chambers, and 

 they occupy, in the form of a fine powder, little vesicular cavities in the tissue of 

 the pinnules. The epidermis of these pinnules now serves as the wall of the 

 cavities, i.e. of the sporangia. 



Each plant in the group of the Ophioglossese exhibits two kinds of frond: the 

 one kind develops no spores and has the appearance of a green foliage-leaf; the 

 other produces sporangia, which are arranged either like bunches of grapes or in 

 spikes consisting almost entirely of the sporangia (see fig. 189^). A similar 

 arrangement may be observed also in many Ferns belonging to other divisions, as, 

 for instance, in the genera Allosorus, Struthiopteris, and Blechnwm, representatives 

 of which occur in the European Flora as well as in others. In other cases, such as 

 the Flowering Fern {Osmunda regalis), for example, sporangia are only formed on 

 the upper portion of a frond, whilst the lower segments are foliaceous. A very 

 peculiar form is that of Rhipidopteris peltata, a fern indigenous in the mountainous 

 regions of Mexico (see fig. 189 *). Besides the flat, fan-shaped fronds which produce 

 no sporangia, other fronds shaped Kke funnels or shallow bowls are developed, and 

 the spore-cases are produced from the epidermal cells in the hollows of these 

 fronds. 



In the last case it is worthy of note that the sporangia are formed on the upper 



