196 



Transactioiis. 



rhizome-apex they have abundant contents. The sieve tubes are very 

 much smaller than in L. voluhile. There is a much scantier development 

 of small-sized flanking xylem elements, but the main metaxylem tracheides 

 are large (fig. 14). The dorsiventral structure of the main rhizome stele 

 passes over into the lower regions of the aerial stems, but this is more or 

 less quickly replaced there by the stellate or radial form. The largest aerial 

 branches not infrequently possess towards their base a larger stele than do 

 t!ie main rhizomes. At the base of one such stem I found as many as 

 twenty-one protoxylem groups, whereas the number in a full-grown rhizome 

 is generally about seventeen or eighteen. In this connection reference may 



be made to Jones's remark with 

 regard to the vascular structure 

 in the stem of L. obscuruni 

 (21, p. 32). He says, '' In any 

 hypothesis which endeavours to 

 explain the dorsiventral struc- 

 ture in the creeping stems the 

 fact must not be overlooked 

 that the banded structure is 

 well marked in the stem of the 

 erect-growing species of L. 

 ohscurum." There is no doubt 

 that L. obscurum, as also L. 

 densum, must be removed from 

 Pritzel's Cernua section and 

 placed in the Clavata section. 

 L. densum is not an erect-growing 

 plant, as stated by Pritzel, but is 

 plagiotropic, and it is practically 

 certain, judging from the spore- 

 ling plants which I have found, 

 that the prothallus belongs to one or other of the deep-growing, massive 

 types. Mr. Cheeseman has informed me that in both Bretton's and Asa 

 Gray's manuals of the flora of the United States of America L. obscurum 

 is described as possessing subterranean creeping " rootstocks." Moreover, 

 Spessard (26) has found the prothallus of this latter species to be of the 

 L. clavatum type. Even if the material of L. obscurum examined by Jones 

 was not that of the main creeping stem but of the lateral branch, the fact 

 of its dorsiventral structure would not be significant, since, as I have 

 shown in L. densum., this frequently persists for some distance up a 

 branch after having been carried over from the main stem at the point 

 of forking. 



The main subterranean rhizome of L. fastigiatum is much shorter than 

 that of the other three species, and is more slender than that of L. densum 

 or L. scariosum. Two characteristic features of its vascular tissues are tlie 

 small size of the xylem elements and the sieve tubes, and also the wide 

 double-zoned peri cycle. The nature of the pericycle in this species will be 

 seen from fig. 15. It consists of two distinct zones, the inner of which is 

 from four to five cells wide, and shows abundant contents, and stains a 

 reddish-brown with safranin, and an outer zone three to four cells wide, 

 also showing abundant contents, but staining purple with the haematoxylin 

 Evidently this wide pericycle has been developed as a storage tissue, and 

 the stored substance in both zones appears to be starch, although the reason 



Fig. 14. 



— Lycopodium densum. Transverse 

 section of portion of stele of main 

 rhizome. X 137. 



