THE GENUS EQUISETTJM. 



171 



rogenous tissue had given rise to spore-mothers and too little to the formation of tapetal 

 cells, and, as a consequence, the resultant spores produced only depauperate plants. A 

 series of observations in (his direction might prove interesting in connection with Bower's 

 theory of the origin of the sporophyte. PI. 26, figs. L2 and L 3, sufficiently illustrate the 

 external features of the young spbrophytes of A T . limosum, and show that they do not 

 differ essentially from those of /*,'. /ii<'nt<//<\ excepl in their greater delicacy, and in the 

 Smaller number of segments formed in the first shoot. 



Turning our attention now to the internal development of E. hiemale, we find that, 

 in the first axis of the young sporophyte there is a gradual transition from the typical 

 arrangement of the tissues in the root, to that obtaining in the base of the young stem. 

 The central cylinder of the first shoot makes its appearance as an Unbroken tube of 

 reticulated tracheites. There are no typical protoxylem elements, although the internal 

 tracheides are formed first. The primitive vascular axis, in fact, starts out with a similar 

 organization to that which is found subsequently to recur at the nodes. The center of the 

 vascular tube is occupied by parenchyma, which may be considered in the light of what is 

 to follow, as belonging to the pericycle. Outside the ring of reticulated vessels occurs a 

 zone of phloem, terminated by a typical endodermis, with the usual radial lignified bands. 

 The rudiment of the second shoot causes an interruption in the continuity of the vas- 

 cular cylinder of the primary axis above its point of origin which disappears again, at 

 about the level of exit of the leaf-traces from the central cylinder. The latter do not 

 cause any gaps in the vascular ring as they pass off, and it is only at a measurable dis- 

 tance above their points of origin, that the so-called foliar lacunae make their appearance. 

 The first shoot-bud originates between two leaf-traces as do all the subsequent ones. The 

 foliar lacunae described above, if they may so be called, divide the fibrovascular tube into 

 three distinct strands which alternate with the leaves. These three strands differ from 

 the woody ring below in having typical protoxylem elements, which lie in more or less 

 characteristic carinal lacunae. They are surrounded by a common external endodermis. 

 At a point two or three millimeters above the exit of the leaf-traces of the primary whorl, 

 a cell makes its appearance in the midst of the internodal bundles, which is characterized 

 by having endodermal markings on its walls ; still higher up this gives place to a radiating 

 group of cells, which have endodermal dots about the middle of their radial walls. Just 

 above the exit of the leaf-traces of the next whorl, that is at the base of the next inter- 

 node, the internal endodermal elements disappear. They again become evident at a short 

 distance above this point, only to disappear again above the exit of the leaf-traces of the 

 next internode. These variations recur in all the segments of the first shoot and need not 

 be further described. In the higher internodes, however, the internal endodermis shows a 

 tendency to unite with the outer one, just below the continuous rings of reticulate vascu- 



