FLORIN: SYSTEMATICS OF THE CYMNOSPERMS 325 



of plants in groups of different orders was understood to be due to divergent 

 evolution from a common ancestor. Classification should express not only struc- 

 tural resemblances, but also relationships by descent. Under the stimulus of 

 Hofmeister's (1851) and Darwin's epoch-making works, the period saw great 

 advances in several fields which affected the conceptions of the position and re- 

 lationships of the gymnosperms. The continued progress of microscopy made 

 possible a better knowledge of the internal structure of plants and the ultiliza- 

 tion of such characters in taxonomy. Strasburger (1875, 1879b), who from the 

 middle of the 1870 's was the leader in the field of cytology, demonstrated that 

 cells were formed directly out of previously existing cells, that there was no 

 free formation of cell nuclei in Sclileiden's sense, and that the nucleus could 

 only derive from a pre-existing nucleus. Another foundation stone of modern 

 cytology had been laid. 



To Nageli (1858) is due the credit of being the first to give an account of 

 plant histology from the developmental point of view. He (1878) believed (cf. 

 Hofmeister, 1867, 1868) that apical growth was fundamentally of the same na- 

 ture in vascular cryptogams and phanerogams, and that it originated in an 

 apical cell. Hanstein (1868), however, thought that in the shoot apices of higher 

 plants growth took place in three histogenetic layers, the dermatogen, periblem, 

 and plerome, each derived from a single cell or a group of initials. Objections 

 to Hanstein's tissue classification were raised by Strasburger (1872), and in 

 1877 De Bary found the histogen theory unsatisfactory as far as the gymno- 

 sperms were concerned. Sachs (1874) distinguished* between epidermal, fibro- 

 vascular, and fundamental tissues, all believed to derive from a uniform api- 

 cal meristem. 



Van Tieghem paved the way for a synthetic grouping of the facts relating 

 to the primary arrangement of the vascular system of stem and root. In 1870 

 and 1872 he laid the first foundation of his stelar theory and pointed out that, 

 essentially, root and stem consist of a central cylinder surrounded by a cortex. 

 The latter is bounded on the inside by the endodermis, while the periphery of 

 the central cylinder is marked by the pericycle bordering on the endodermis. 

 The central cylinder is differentiated into a pith of parenchyma, differing in 

 origin from that of the cortex, and vascular bundles separated by medullary 

 rays. 



Nageli (1858) regarded Schimper-Braun's purely formal theory of phyllo- 

 taxis as unsatisfactory, and brought up the question of the relation between 

 phyllotaxis and vascularization. Like him, Geyler (1867-1868) found that in 

 conifers and related groups all vascular bundles are common to stem and leaf 

 and join lower bundles in the stem. As to Ginkgo, they believed that the two 

 strands of each leaf trace fuse into one bundle in the central cylinder, and that 

 this bundle is then united with a lower bundle. In 1868 Hofmeister propounded 

 a mechanistic theory of phyllotaxis which was further developed by Schwen- 

 dener (1878). Mettenius (1860) examined the peculiar course of the leaf traces 

 in stems of cycads. 



A thorough investigation into the origin, structure, and activity of the cam- 

 bium was carried out by Sanio (1863, 1873-1874), who, contrary to Nageli's 

 opinion, found that a continuous procambium ring is formed directly in the 

 primordial meristem of the shoot apex, and further that the xylem and phloem 



