322 SPOROPHYLLS OF EQUISETUM 



which the various tissues and organs of the plant are formed. 

 The very rudimentary vascular bundles are arranged around a 

 large pith (Fig. 233). They rarely show secondary growth 

 through the activities of a cambium, although this was a marked 

 feature of many of the extinct species. These bundles lie directly 

 below 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 con- 

 stitute the principal mechanical tissue of the stem, while the 

 stomata and the 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 formed only upon special leaves, sporophylls, that are 

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

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

 borne upon a special branch that does not contain chlorophyll, 

 though variously colored, and that withers away as soon as the 

 spores are shed (Fig. 232, h). In other cases the strobilus ap- 

 pears at the tip of the ordinary green shoot. This variation is 

 doubtless associated with the season of the year at which the 

 strobili appear. The species characterized by the special branches 

 produce these early in the spring when the temperatures are 

 not favorable for the work of the green shoots. Note in this 

 connection the significance of the coloration. Species with the 

 strobili upon the ordinary plants form these structures later 

 in the season when the temperatures are favorable for photo- 

 synthesis. The development of the sporangia upon specialized 

 sporophylls that are grouped together at definite points on the 



