306 SPORES OF EQUISETUM 



each ridge of the stem and run downward through the internode 

 to the next lower node, where they divide into two equal parts. 

 One of these branches joins a similar branch from the adjacent 

 bundle and continues straight down to the next node, where the 

 branching is repeated. The rudimentary bundles of the leaves 

 also join on to the bundles of the stem at the node. Owing to 

 this regularity of branching, the leaves and vascular bundles 

 alternate in each succeeding node and the vascular system assumes 

 a cylindrical form composed of oblong six-sided figures. Thick- 

 walled stereome cells are developed in the ridges and constitute 

 the principal mechanical tissue of the stem, while the stomata 

 and chlorophyll apparatus are largely confined to the grooves. 

 The stems are very light, owing to absorption of the larger part 

 of the pith and certain regions of the cortex (Fig. 233, a). 



(a) The Spore-bearing Leaves or Sporophylls. — The sporan- 

 gia are only formed upon special leaves, sporophylls, that are 

 arranged in a compact cone or strobilus (phi. strobili) at the end 

 of the stem (Fig. 232, sp). In some species the strobilus is 

 borne upon a special branch that withers away as soon as the 

 spores are shed (Fig. 232, b) and in other cases the strobilus 

 appears at the tip of the ordinary green shoot. The develop- 

 ment of the sporangia upon specialized sporophylls that are 

 grouped together at definite points on the plant is one of the 

 significant departures that appears in the evolution of the plant. 

 This distribution of labor that is seen in the setting aside of cer- 

 tain groups of leaves for spore production will appear in all the 

 succeeding groups ; and the arrangement and structure of the 

 sporophylls will steadily become more and more complex until a 

 point is reached where they are popularly called a flower, al- 

 though this term can be just as correctly applied to the strobilus 

 appearing in Equisetitm and succeeding groups. The sporophylls 

 of Equisetitm originate at the nodes, as in the case of the scale 

 leaves, but as they enlarge the apical portion spreads out like a 

 shield. The internodes do not elongate and separate the sporo- 

 phylls to any considerable extent, consequently the shields become 

 six-sided through contact with the adjacent sporophylls. 



(b) Character of Sporangia and Spores. — The sporangia ap- 



