EQUISETIN& 367 



sporangia, when mature, appear as nearly cylindrical sacs 

 attached by one end to the under surfaces of the peltate 

 scales (sg, Fig. 251, B); they open at maturity by a slit along 

 the inner face i.e., the side next to the pedicel of the pel- 

 tate scale. 



48O. In their development the spores acquire three con- 

 centric coats, and as they approach maturity the outer one, 

 which has previously become spirally thickened, splits from 

 two opposite points into four narrow spiral filaments, which 

 are united with one another and the spore at a common 

 point. These filaments are hygroscopic, and they roll and 

 unroll with the slightest changes in the moisture of the air : 

 when moistened they wrap tightly 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 considerably, and are doubtless useful in empty- 

 ing the sporangia after dehiscence hence they have been 

 called Elaters. 



481. The spores germinate soon after falling upon water 

 or moist earth ; they first enlarge, and then divide by a par- 

 tition into two parts of unequal size, the larger of which 

 contains chlorophyll granules, while the smaller one is color- 

 less ; the latter grows rapidly into an elongated root-hair. 

 The larger cell divides first into two cells, and then usually 

 one of these divides again, and so on, giving rise to a simple 

 prothallium, composed of a single layer of cells ; this en- 

 larges and increases in size, until it reaches the stage in 

 which it bears the sexual organs (paragraph 475). 



482. Tissues. The epidermis is remarkable for the large 

 quantity of silica which it contains, mainly in the outer 

 walls of the cells. The epidermal cells are mostly narrow 

 and elongated, and are arranged in vertical rows. The sto- 

 mata, which are present in all the chlorophyll-bearing parts 

 of the plant, are arranged with more or less regularity in 

 longitudinal rows ; on the stem they occur in the channels 

 between the numerous ridges. They resemble pretty closely 

 the stomata of the Phanerogams in their structure. The 

 fibro- vascular bundles of the stem are disposed in a circle, as 

 seen in a cross-section, and they run through the fundu- 



