GROUP III. PTERIDOPHYTA : E^UISETIfOE. 



383 



are nearly spherical when ripe. Each spore has two coats, exospore and 

 endospore, and originally a perinium is present. The perinium, as it 

 developes, becomes irregularly thickened in such a way that, when the 

 thin portions are destroyed, the thickened portions remain as four fila- 

 ments, the elaters, all attached at one point only to the spore. These 

 elaters are very hygroscopic. When the air is dry they expand, and 

 stand out stiffly from the spore ; when moistened, they suddenly roll up 

 spirally round the spore. The spores become entangled by their elaters, 

 so that when set free from the sporangium a number of the spores fall to 

 the ground and germinate near together. 



The roots are all adventitious, though a short-lived primary root is 

 developed. They 

 are developed at 

 the nodes of the 

 rhizome. 



General Histo- 

 logy. A striking 

 feature in the 

 anatomy of the 

 stem is the pre- 

 sence of large, 

 mainly lysigen- 

 ous, air-cavities : 

 thus, in some 

 species, the rhi- 

 zome has a large 

 centra] cavity 

 in each inter- 

 node (Fig. 230 C, 

 a) ; a similar 

 cavity is present 

 in the internodes 

 of the aerial 

 shoots of nearly 

 all the species 

 (Fig. 230 A, ); 

 the central cavi- 

 ties of successive 

 internodes are 



shut off from each other by diaphragms at the nodes (Fig. 227 n) : a 

 series of similar cavities occurs always in the cortex, alternating with the 

 vascular bundles internally and with the surface-ribs externally, hence 

 termed vail ecu 1 nr cavities (Fig. 230 1>) ; finally, in connexion with each vas- 

 cular bundle, there is a large cavity, the carinal cavity (Fig. 230 c), extend- 

 ing, like the others, from one node to another. 



The growing-point of the stem, and of its branches, is apical, and has a 

 tetrahedral apical cell (Fig. 229). 



In the aerial shoots (except specialised fertile shoots of E. arvenie, etc.) 



FIG. 229. Growing-point of the stem of ISquisetum. arvensc, in 

 longitudinal section; t apical cell; '" successive segments; 

 p anticlinal segment-wall ; m wall dividing the segment into an 

 upper and a lower cell ; pr penclinal wall dividing the segment 

 into an inner and an outer cell ; fff" successive whorls of leaves ; 

 9 initial cell of a lateral bud. (After Strasburger: x 240.) 



