14 INTRODUCTION. 



The differentiation of the meristematic apex of the stem of the Gymnosperms ^ 

 shows a varying character, which couples it on the one hand with that which 

 appears in the typical Angiosperms, on the other hand with that in the Lycopo- 

 diacece, while in Araucaria brasiliensis, also in A. Cunninghami, Dammara, and 

 Cunninghamia, the dermatogen, periblem, and plerome remain clearly distinguished, 

 in the extreme apex ; in the Abietinece and Cycas these layers run together into 

 a common initial group, which occupies the extreme apex ; a separation into the 

 three layers first appears at some distance beneath this in Cycas, and more clearly in 

 the Abietinea). Ephedra is specially interesting, for here, in the same species 

 (E. campylopoda) and apparently fluctuating in the same shoot, the character varies 

 between the two extremes described. At one time there is a dermatogen-layer, 

 sharply defined throughout its whole course, covering the two inner layers which 

 in the extreme apex are more or less clearly separate ; in other cases both merge 

 with the dermatogen into a common superficial initial group. A series of similar 

 cases, some corresponding to Araucaria, others approaching the other extreme, were 

 made known by Strasburger's researches on Taxus, Podocarpus, Saxegothea, 

 Ginkgo, Thuja, Cupressus, Sequoja, and Cryptomeria. 



Here, as in the Angiosperms, the dermatogen and periblem alone take part in 

 the origination of leaves and normal lateral shoots, and in most cases also in the 

 same fashion as there. In the Abietinese, however, there occur also divisions parallel 

 to the surface in the dermatogen of the young leaf. 



III. As already intimated, the differentiation of the meristematic apex of the 

 LycopodiacecB ^ corresponds closely w'ith that of the Gymnosperms. The extreme 

 apex of the stem is occupied by a group consisting of 2-4 prismatic cells with their 

 longer axis at right angles to the surface. This is the initial group for the periblem 

 and dermatogen, or rather for a superficial layer which corresponds to the latter. 

 All these initial cells divide by walls perpendicular to the surface, and the products of 

 this division, as they are removed from the apex by the advance due to the growth 

 of the latter in length, divide again parallel to the surface, forming thus the initial 

 layers of dermatogen and periblem. A plerome-cylinder, which is limited laterally by 

 the periblem, elongates independently through the activity of an initial group or single 

 cell of its own, which occupies the centre of its conically tapered apex, and lies imme- 

 diately beneath the initial group of the outer layers. As Hegelmaier has already as- 

 serted, conditions are to be found, which point to the origin of the initial cells of the 

 plerome from the common initial group at the surface of the apex (by transverse divi- 

 sion). It is thus possible that a fluctuation of the definition of layers occurs here simi- 

 lar to that in the Coniferae. The formation of the leaves starts in the Lycopodiaceas 

 from one cell of the outermost (dermatogen) layer, which, after arching outwards, 

 divides first parallel to the surface, then further. The differentiation of the meristem- 

 atic apex of the root is, according to the investigations of Strasburger and of Bruch- 

 man, similar in the Lycopodiacese to that in the first type of the Angiospermous roots. 



' Strasburger, /.c. p. 323, Taf. 22, 23, 25. — Pfitzer, in Pringsheim's Jahrb. VIII. p. 56.— [Dingier, 

 Botan. Zeitg. 1882, p. 795-] 



^ Cramer, Pflanzenphysiol. Unters. Heft III. p. 10. — Strasburger, Coniferen, p. 336. — Hegel- 

 maier, Bot. Zeitg. 1872, p. 798 ff., 1874, p. 773. — Bruchmann, Ueber Wurzeln von Lycopodium uiid 

 Isoetes, Jena, 1874. — Compare also Russow, Vergl. Unters. p. 176. 



