PTEBIDOPUTTA. 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. 

 elater, 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 AAVvV* 

 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 Eguiseium, 



