1907] STOKEY—ROOTS OF LYCOPODIUM PITHYOIDES ■ 61 



of the middle lamella of the inner cells. It is so delicate that any 

 pressure sufficient to section the sclerenchyma sheath is almost 

 certain to cause a splitting of the middle lamella. Although the two 

 regions are rather sharply defined, there are always a few cells in the 

 transition stage. The thin-walled cortex is limited by the endoder- 

 mis, which although not sharply differentiated can be easily recognized 

 by its suberized walls. 



In the genus Lycopodium there is great variation in the type of 

 stele found in the root. Russow (8) has described that of L. annoti- 

 num and Z. complanalum as being almost identical with that of the 

 stem, but the stele of L. Selago and L. inundatum as being strikingly 

 different. L. pithyoides is similar to L. Selago. The vascular strand 

 is collateral, with a crescent-shaped xylem, the phloem lying between 

 the horns of the crescent. The protoxylem is found regularly occupy- 

 ing the horns, but occasionally it spreads along the outer region of the 

 metaxylem. There is no evident differentiation into protophloem 

 and metaphloem either in root or stem. Russow in his discussion of 

 L. Selago regards the parenchyma region outside the xylem as belong- 

 ing to the phloem, and consequently classes the bundle as concentric. 

 This indefinite region may be one or two cells thick or may be 

 altogether lacking, and it seems better to regard it as pericycle rather 

 than as phloem. It consists of small thin-walled parenchyma cells 

 containing protoplasm and nuclei, with none of the wide-mouthed 

 thick-walled cells which characterize the phloem region. The shape of 

 the xylem strand varies somew^hat in different parts of the root. The 

 origin from the stele is seen in jigs. 13 and 14. It is connected with 

 two of the xylem strands and one of the phloem; this arrangement 

 is shown in the primordium stage shown in jig. 4. As the root leaves 

 the stele of the stem, the xylem strands move together, forming a U- 

 shaped figure, with the closed end toward the center of the stem {figs. 

 15 and 16.) As the root passes down the stem, the xylem becomes 

 massed together, so that when the root leaves the stem it has the form 

 shown in fig. 77, which is the arrangement characteristic of external 

 roots. The behavior of the xylem during forking of the root is shown 

 in figs. 18, ip, 20. The xylem divides at the place where the two 

 strands had previously united, and each strand rapidly assumes the 

 characteristic crescent-shaped form. 



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