362 Transactions. 



the leaf-traces are very numerous and irregularly disposed, and their loose 

 aggregation at the centre of the stem axis is prior to the development of a 

 regular plerome cylinder ; moreover, after the development of the plerome 

 cylinder, the leaf-traces attach themselves quite indiscriminate!}^ to any 

 point on its periphery. In the young plant of L. Billardieru L. voluhile, 

 and L. scariosum, however, the development of the leaf-trace system is much 

 subsequent to that of the regular plerome cylinder, and the leaf -traces attach 

 themselves very definitely to the compact protoxylem groups of the cylinder. 

 In these latter species also the leaf-traces are far less numerous than in the 

 two former species ; especially is this so in the young plant of L. Billardieri, 

 where the leaves are few in number, arranged in two and three orthostichies, 

 and are very large and broad compared with the size of the plant. We may 

 thus regard the type of vascular arrangement in the young plant of L. cernuum 

 and L. laterale as being less specialised than that in L. BUlardieri, L. voluhile, 

 and L. scariosum, and representing an earlier stage in the development of 

 the Lycopodium stele, the more definite arrangement as shown in the three 

 latter species being made possible by the diminu^tion in number of the leaf- 

 traces consequent on the adoption by the prothallus and young plant of 

 subterranean habits. 



Development in Complexity of Stem Stele. 



The development in complexity of the loose or " mixed " type of stem 

 stele of L. cernuum and L. laterale shows no features worthy of special 

 note. Throughout the life of the plant of both species the vascular tissues 

 of the stele show a very indefinite conformation, the phloem and xylem 

 elements being loosely grouped together in irregular bands and islands 

 whose forms are constantly changing. The numerous leaf-traces attach 

 themselves indiscriminately to the periphery of the stele. There is no 

 rearrangement of the stelar tissues preparatory to branching. In the 

 smaller branches there is no definite radial arrangement of the vascular 

 tissues, and no constant relation to be observed subsisting between the 

 leaf-trace system and the number of protoxylem groups. In the strobili 

 of L. laterale, however, where the leaves are arranged in six orthostichies 

 in alternate whorls of three, the arrangement of vascular tissues is con- 

 sistently and definitely triarch, and there is a constant relation between 

 the leaf-trace system and the number of protoxylem groups. 



The development in complexity of the definite compact type of stem 

 stele of L. Billardieri, L. voluhile, and L. scariosum was closely followed, 

 abundant material being to hand for the purpose. In the case of L. voluhile 

 and L. scariosum, features were noticed which may possibly throw some 

 light on the well-known " parallel " or " dorsi-ventral " arrangement of 

 vascular tissues of which these two species are such excellent examples. 



In L. Billardieri, in plants about 2 in. high, showing from 10-12 well- 

 formed leaves, the stem stele consists of two compact xylem groups and a 

 single intermediate phloem group. The number of xylem groups later 

 becomes three by the splitting of one of the two original groups, and the 

 phloem extends between these groups. The disposition of vascular tissues 

 in the stem is definite throughout the life of the plant, the individual elements 

 of xylem and of phloem being mutually coherent. 



Branching of the stem is dichotomous ; it is not restricted to one plane. 

 In the main stem there is no particular rearrangement of the xylem and 

 phloem plates preparatory to branching. In the smaller branches the 

 plates of tissue very often arrange themselves more or less at right angles 



