PTERIDOPRYTA. 155 



the spores are ripe. The modified spore-bearing leaves are 

 peltate in form, and collected into cone-shaped clusters ; in 

 consequence of pressure each becomes hexagonal in outline ; 

 on the under surface of each peltate leaf, or scale, there 

 arise several sac-shaped sporangia, which open at maturity 

 by a slit on the side next to the pedicel, and allow the 

 escape of the spores. " In their development the spores 

 acquire three concentric coats, and as they approach matu- 

 rity the outer one, which has previously become spirally 

 thickened, splits, from two opposite points, into narrow 

 spiral filaments, which are united with one another and 

 the spore at a common point. These filaments are hygro- 

 scopic, and they roll and unroll with the slightest changes 

 in the moisture in the air; when moistened, they tightly 

 wrap around the spore ; but when dry, they unroll, and 

 become more or less reflexed. By the changes of position 

 which they undergo, they move the spores very consider- 

 ably, and are doubtless useful in emptying the sporangia 

 after dehiscence, hence they have been called elaters" (Gr. 

 elat&r, driver). The epidermis of the Equisetinese contains 

 a large amount of silica. The stomates are arranged in 

 longitudinal rows — in the channels between the ridges. 

 The fibro-vascular bundles are arranged in a 

 circle surrounding the central cavity. Each 

 bundle passes isolated down through the inter- 

 node, and at the node divides into two short AAAV^ 

 branches, which unite right and left, with cor- 

 responding branches of other bundles, thus 

 forming the alternating bundles of the next 

 lower internode (Fig. 267). The single bundle 267 

 from each leaf is applied at the point where the descending 



Fig. 267. Diagram, showing the course of the fibro-vascular bundles in the stem 

 of £guiseiujn. 



