INTRODUCTION. 



during all their further growth are divided only perpendicularly to the surface, so 

 that the dermatogen-layer is continuous as a single layer also over the branches. 

 The plerome-cylinder of the mother shoot, as far as investigation extends, takes no 

 part in the origination of a branch. In the lateral shoots thus founded the separa- 

 tion of the meristematic mass, covered by dermatogen, into periblem and plerome 

 first appears after some time. Both in this case originate from a common initial 

 group, which is derived from the periblem of the mother shoot. 



In the punctum vegetah'onis of the root of the Angiosperms ^ the same differen- 

 tiation of the meristem often appears, as in the stem, but sometimes much more 

 definitely. It should be described in the same terms as the latter, so far as the 

 correspondence is exact. To the meristem, from which springs the body of the 

 root, is added in all roots the conical cap, made up of layers of cells, which is known 

 as the Root-cap (calyptra). This covers the meristematic puncitim vcgetalionis, and 

 is increased by it, according as the cells on its outer surface die ofi". Since this 

 accession originates in certain cases from a special layer of meristem, the latter is, 

 according to Janczewski, to be distinguished as the calypirogeti. As has already been 

 said above, we cannot here discuss what 

 genetic relation this bears in the main root 

 of the embryo to the first meristem-cells 

 of the hypocotyledonary stem. We must 

 also take no notice here of the origination 

 of the layers of meristem of lateral roots, 

 our knowledge of which is for the most part 

 due to Janczewski. 



At the active punchan vegetationis of 

 the roots of the Angiosperms, which has 

 already begun to grow in length, four dif- 

 ferent cases of differentiation, which vary 

 according to the species or groups, have 

 been made known to us by Janczewski : 



1. The meristem at the apex is diffe- 

 rentiated into four sharply marked off layers : 

 plerome-cyhnder, periblem, dermalogcn, and 

 lastly the calyptrogen-layer, which covers the 

 latter, and which soon disappears owing to 

 the short duration of its activity. This diffe- 

 rentiation is found only in two INIonocotyledonous water-plants, viz. Hydrocharis and 

 Pistia stratiotes^ (Fig- 2). 



2. Sharply defined plerome-cylinder, and calyptrogen-layer. Between the two, at 

 the apex of the piincium vegetationis, is an initial group only one layer of cells thick, 

 which splits immediately behind the apex into periblem and dermatogen (i. e. cortex 

 and epidermis). This is the case in most of the Monocotyledons which have been 



Fig. 2.— (450) Pistia stratiotes. Median longitudinal sec- 

 tion through a young lateral root, h Root-cap, including 

 calyptrogen-layer ; e dermatogen ; / c outer layer of the 

 plerome-cylinder (pericambium) ; v foundation of an axile 

 vessel in this. Between e and / <: the periblem, which consists 

 at the apex of only a single layer. 



1 [See further Holle, Botan. Zeitg. 1876, p. 241; 1877, p. 537. — Eriksson, Botan. Zeitg. 1876, 

 p. 641. — Schwendener, Scheitelwachsthum d. Phanerogamen-wurzeln, K. Acad. Wiss. Berlin. 1882; 

 Botan. Zeitg. 1882, p. 687.— Flahault, Ann. Sci. Nat. ser. 6, Bot. torn. VI. pp. 1-229.] 



2 [Kubin, Hanstein's Abhandl. Bd. III. Heft 4, 1878.] 



