INTRODUCTION. 489 



where a single cell (not, however, of preponderating size) occupies the apex, and the 

 arrangement of the superficial cells of the punctum vegetationis appears to point to it 

 as the primary mother-cell, it is nevertheless by no means to be assumed that all the 

 cells, and especially the internal mass of the primary meristem, have proceeded from 

 it. The primary meristem of the punctum vegetationis consists of a large number of 

 usually very small cells, more or less evidently disposed in concentric layers ; an 

 outer single layer, the dennatogen, may be recognised in Angiosperms as the imme- 

 diate continuation of the epidermis of the older parts, and is continuous even over 

 the apex of the punctum vegetationis. Beneath it lies a second meristematic tissue, 

 [the periblem'], consisting usually of a few layers of cells, which covers the apex and 

 passes lower down into the cortex ; this envelopes a third inner mass of tissue, [the 

 plerome^ terminating beneath the apex as a single cell ^ {Hippuris^ &c.) or as a group 

 of cells ; and out of it proceeds either an axial fibro-vascular body (in roots, and in 

 the stems of water-plants), or the descending limb of the fibro-vascular bundles. In 

 harmony with this the root-cap does not proceed, as in Cryptogams, from transverse 

 divisions of an apical cell, but arises, on the contrary, in Gymnosperms from a 

 luxuriant growth of the layers of periblem of the root and from their splitting away 

 towards the apex, and in Angiosperms from a similar process in the dermatogen, 

 or from a special meristematic layer the calyptrogen ^. Even the first rudiments of 

 lateral structures, leaves, shoots, and roots, cannot be traced back in Phanerogams 

 to a single cell in the same sense as in Cryptogams. They are first observable as 

 protuberances consisting of a few or a larger number of small cells ; the protuberance 

 which is to form a shoot or a leaf shows, even when it first begins to swell, an inner 

 mass of tissue which is connected with the periblem of the generating vegetative 

 cone, and is covered over by a continuation of the dermatogen. 



The normal Mode of Branching at the growing end of the shoot, leaves, and 

 roots, is, with few exceptions, monopodial ; the generating axis continues to grow 

 as such, and produces lateral members (shoots, lateral leaf-branchings, lateral roots) 

 beneath its apex. Some cymose inflorescences appear however to be the result of 

 dichotomous branching, and it is possible that in the Cycadeae also the branching 

 of the stem and leaves may be dichotomous. The monopodial branching of the 

 axes is usually axillary ; i. e. the new rudiments of shoots appear above the median 

 plane of very young (but not necessarily the youngest) leaves, in the angle which 

 they form with the shoot, or somewhat above it. In Gymnosperms every axil of 

 a leaf does not usually produce a shoot; sometimes (in Cycadeae), the branching 

 of the stem, as in many Filicineae, is reduced to a minimum. In Angiosperms, on 

 the contrary, it is the rule that every axil of a foliage-leaf {i. e. one not belonging to 

 the flower) produces a lateral shoot (sometimes even several side by side or one 

 above another); but commonly the axillary buds, once formed, are inactive, or 

 develope only at later periods of vegetation. In addition to the above-mentioned 



^ As in so many other respects, here also Isoetes shows an affinity to Phanerogams, as is 

 evident from Nageli and Schwendener's researches on the apical growth of roots. (Compare Nageli's 

 Beitragen, 1867, Heft. IV. p. 136.) 



2 See Hanstein, Bot. Abhandl. Heft I, and Reinke, Gottinger Nachr. 1871, p. 533. [Janczewski, 

 L'accroissement terminal des Racines, Mem. soc. nat. de Sci. de Cherbourg, 1874, and Ann. d. Sci. 

 nat. ser. 5, t. XX.] 



