DEVELOPMENT OF PLANTS 337 



walls and stomata can be obtained by treating the tissue as noted 

 in the diatoms. The arrangement of the leaves and branches and 

 the distribution of the vascular bundles and other tissues presents 

 a mathematical regularity unexcelled in any plant. As in the 

 Bryophyta, the elongation of the stem is effected by a single 

 apical cell which cuts off with extreme regularity the cells from 

 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 (Fie. 



233> (^)- 



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

 are formed only upon special organs, probably leaves or sporo- 

 phylls, 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 stro- 

 bilus appears at the tip of the ordinary green shoot. This vari- 

 ation 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 



