VASCULAR TLOWERLESS PLANTS. 85 



indusium i; c one of the thecse taken from the cluster, and 

 more highly magnified. It consists of a flattened bag of cellu- 

 lar tissue, which is partially surrounded by an elastic ring of 

 cells, annulus, having a special structure. The walls of the 

 cells of the annulus are much thicker than the walls of the cells 

 which they surround ; and, as the annulus has a continual 

 tendency to straighten itself, when the theca becomes dry and 

 the spores matured, it is suddenly torn open by the elastic 

 straightening of the annulus, as shown at d, and the spores e, 

 contained in its cavity, scattered. This beautiful contrivance 

 of nature may be seen in operation in the ripened sori situated 

 on the back of any dorsiferous fern. 



121. The thecse attached to the frond beneath the indu- 

 sium appear to produce all the peculiarities of its rupture, by 

 their different modes of development. In Polystichum acrosti- 

 choides, the marginal thecse of each soVus are the first to de- 

 velop, and the indusium is thus ruptured from the paren- 

 chyma and the epidermis, all round the margin, whilst it 

 remains attached to the frond by its centre. In Woodsia, on 

 the contrary, the central thecse of the sori are the first to de- 

 velop and effect a corresponding rupture in the cuticle, for 

 the indusium is ruptured at its centre and forms a sort of 

 cup-shaped involucre around the thecse, being continuous with 

 the epidermis of the frond at the margin. Sometimes the in- 

 dusium is a mere fold of the margin of the frond, as in Pteris 

 and Adiantum. 



122. "We stated that the thecse or spore-oases of ferns were 

 developed from the parenchyma of the frond. In some genera, 

 the sori, or clusters of thecse, are developed at the expense of 



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