THE GENUS EQUISETUM. 173 



becoming subsequently fused gave rise to a continuous outer and a continuous inner 

 endodermis. It is not unreasonable to expect to find some evidence of this in the 

 young stem. If anything may be argued from these embryological results, the primi- 

 tive condition would j-eem to have been one in which there was both an -outer and 

 an inner endodermis. This subject, however, will be more satisfactorily discussed at 

 a subsequent stage. 



Having examined the external and internal development of the young sporophyte 

 of Equiseta we may now turn our attention to certain features of the adult, wliich are 

 either in themselves interesting, or which are of importance in coming to any conclu- 

 sions concerning the affinities of the Equisetaceae. 



Photograph 3 (PI. 28, fig. 3) represents a transverse section through the nodal region 

 of the subteri'anean stem of E. hiemale. On the outside of the stem and on the carinal 

 ridges are to be seen certain projecting organs, a, of a parenchymatous nature. These are 

 situated opposite the points where the. leaf-traces leave the central cjdinder to pursue 

 their upward course in the foliar sheaths. They were noticed by Brongniart (o/>. cit.) 

 in £J. litorale, but apparently have not been elucidated since. At h in the same photo- 

 graph may be seen the origin of a root-trace. At c there is present a shoot-bud. This 

 has several roots attached to its lower end which do not appear in the plane of section. 

 Photograph "4 (PI. 28, fig. 4) represents more highly magnified one of the organs referred 

 to above. The sclerenchymatous cortex of the rhizome is here interrupted by a strand 

 of parenchymatous tissue which extends from the leaf-trace outwards. These organs 

 may be conveniently called nodal organs. Their constituent parenchymatous tissue is of 

 a loose and spongy nature, and their organization is quite similar to that of the lenticels 

 which occur on the roots of the higher plants, and we probably shall not go very far 

 astray in considering that these peculiar nodal organs of certain Equiseta have the func- 

 tion of permitting an interchange of gases through the otherwise impervious integument 

 of the rhizome. They make their appearance on the subterranean parts of (juite young 

 plants of E. hiemale as well as in the adult. 



Photograph 5 (PL 28, fig. 5) shows an entire thin transverse section through the 

 nodal region of E. silvaticum. At a are the nodal organs similar to those of E. hiemale. 

 The origin of root-hairs from the epidermis of the rhizome may also be clearly made out. 

 The outer zone of the cortex is parenchymatous, and internal to it occurs a ring of brown 

 sclerenchyma, the continuity of which is broken by the spongy parenchyma of the nodal 

 organs or by root-traces, h. Inside the sclerenchymatous zone occurs a third, which is of 

 a parenchymatous nature and adjoins the bast of the fibrovascular bundles. The central 

 cylinder is seen to give off six processes, five of which are root-bases and one a dormant 

 branch, c. Alternating with these and much smaller are the leaf-traces, d, subtending 



