498 MOSSES AND FERNS chap. 



leaves are dimorphous and arranged in four ranks, like those 

 of most species of Selaginella. 



The structure of the vascular bundle of the leaf is simple. 

 It is concentric in structure, with the central part composed 

 of a small number of spiral and annular tracheids, and the 

 peripheral portion made up of parenchyma, with a circle of 

 scattered narrow sieve-tubes. A definite endodermis cannot 

 be demonstrated. In the species with the leaves all alike both 

 surfaces bear stomata, but in those with decussate leaves the 

 greater part of the upper surface is destitute of them. 



The Root 



The roots of Lycopodium arise, as in other Pteridophytes, 

 in acropetal succession, but with no relation to the position of 

 the other organs. According to Bruchmann adventive roots 

 may arise in L. inundatum, but they have not been observed 

 in other forms. L. selago (Strasburger (lo), p. 259) may 

 serve to show the characters of the root in the genus. The 

 meristem of the apex is clearly differentiated into the initials 

 of the different primary tissues (Fig. 287, C). The dermat- 

 ogen (d) completely covers the apex of the growing point as 

 a single layer. The periblem (pb) is three cells thick; the 

 plerome (pi) terminates in a group of special initials. As in 

 the stem, the plerome alone forms the central cylinder, the peri- 

 blem giving rise only to the cortex, and the structure of the 

 mature root corresponds closely to that of the stem, except for 

 the presence of the root-cap, which has its own initial group of 

 cells (calyptrogen, cal). From the older dermatogen cells are 

 derived, by special walls, the mother cells of the root-hairs (h). 



Van Tieghem ((5), p. 553) states that the secondary roots 

 arise from the pericycle instead of from the endodermis, as in 

 other Pteridophytes ; but Strasburger claims that the so-called 

 pericycle of Lycopodium is really cortical, and does not belong 

 properly to the central cylinder, so that this difference is only 

 apparent. The endodermis itself is not readily recognisable 

 on account of the complete cutinisation of the walls. 



The origin of the root-hairs is somewhat peculiar. From 

 the base of each dermatogen cell a wedge-shaped cell is cut off 

 (Fig. 287, C, h), and this afterwards is divided into two sim- 

 ilar cells, each of which grows out into a unicellular hair. 

 Thus the root-hairs are found in pairs. 



