FLORIN: SYSTEMATICS OF THE GYMNOSPERMS 329 



The development of the gametophytes was gradually elucidated. Bela Jeff's 

 investigations (1891, 1893) of the male gametes in Taxus and conifers were an 

 important advance. Strasburger (1884, 1892) confirmed in the main his ac- 

 count for all the principal gymnosperm groups, and proved that a mature pollen 

 grain of the Pinaccae and Ginkgo as a rule contains, in polar arrangement, two 

 prothallial cells, one antheridial cell, and the vegetative pollen-tube cell, while 

 Taxus and Cupressaceae lack prothallial cells. 



The discovery by Hirase (1895, 1898) that the Ginkgo ovules are fertilized 

 by motile ciliated sperms caused a great sensation. Strasburger (1892) de- 

 scribed the differentiation of the pollen tube into a rhizoidal and a generative 

 part, which also separates Ginkgo from the conifers. The formation and sub- 

 sequent development of the pollen grains in the cycads had already been ex- 

 amined, but Ikeno (1898) was the first to give a full account of the develop- 

 ment of the male gametoi:)hyte (Cycas) and to demonstrate that, here too, the 

 male gametes were spermatozoids with bands of cilia developed from blepharo- 

 plasts. The occurrence of spermatozoids in Zamia and Stangeria was announced 

 by Webber (1901) and Lang (1900) respectively. These discoveries are remark- 

 able events in the history of plant morphology. 



It was known that the pollen grains, like the spores of the vascular crypto- 

 gams, were formed by tetrad division of mother cells associated with a reduc- 

 tion of the chromosome number. Juel (1900) found that in Larix the megaspore 

 mother cell is homologous to the microspore mother cell, and divides in the 

 same way. In the gymnosperms, only the chalazal megaspore generally develops 

 into a prothallium. The cell formation in the female gametophyte of taxads, 

 conifers, and ephedras was described by Sokolowa (1891), and several other 

 works on the development of the female gametophyte and the archegonium were 

 published. The ventral canal cell or nucleus was interpreted as an arrested 

 gamete. Conditions in Gnetum (Lotsy, 1899) proved to differ from those in 

 other gymnosperms by the lack of archegonia and in other ways. 



The nuclear divisions in tissue cells and spore mother cells, and the fer- 

 tilization process were investigated. Strasburger and his students laid the foun- 

 dations of karyology. The problem of reduction division came increasingly to 

 the fore, and the first observations of the chromosome numbers of plants were 

 made. As to the shoot apex, Koch (1891), and others, found that Nageli's api- 

 cal cell theory was not applicable to gymnosperms. Nor had Hanstein's histogen 

 theory proved tenable. 



Van Tieghem (van Tieghem and Douliot, 1886, 1888; van Tieghem, 1891a, 

 1898, etc.) further developed his stelar theory. Sachs's (1874) classification of 

 tissues was increasingly displaced by the new division into epidermis, cortex, and 

 stele, which applied to stem, root, and leaf. The primitive type of central cylin- 

 der, the monostele, consists of a single concentric fibrovascular strand, bounded 

 externally by the pericycle. It may become polystelic by dichotomy. It may 

 also expand and develop a central pith and radiating medullary rays. In the 

 latter case the endodermis and pericycle may become folded in between the 

 bundles, uniting at the inner side of each. This astelic type may in addition 

 be modified by the separate bundles uniting into a more or less complete ring, 

 bounded by a continuous pericycle and endodermis (gamodesmic stele). The 

 steles of the polystelic axis may in their turn form a concentric annular stele 



