334 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



Lignier (1908-1909; cf. Bugnon, 1922) also considered the leaves of the 

 vascular plants to be diphyletic. The simple uninerved leaves of his Phylloideae 

 were thought to be of the nature of emergences, while those of his Phyllineae 

 were dichotomized and cladodified cauloids. The Phyllineae had developed in 

 different directions. In the Macrophyllineae, comprising ferns, pteridosperms, 

 and cj'cadophytes, the leaf dominated tlie axis. In the Microphj'llineae, com- 

 prising cordaites, ginkgoes, and conifers, the axis had got the upper hand of the 

 leaf, and was more branched. An intermediate position was taken by the Meso- 

 phyllineae or angiosperms. The grouping of the gymnosperms corresponds to 

 Sahni's (1920) later division of them into phyllosperms with leaf -borne seeds, 

 and stachyosperms with stem-born seeds. Lignier 's theory produced various 

 further contributions to the debate (Florin, 1938-1945). 



The overtopping theory of H. Potonie (1912) explained the leafy shoots of 

 the higher plants as derived from the dichotomously ramified thallus of algal 

 plants. Overtopping branches formed between them the axis, with infinite api- 

 cal growth, while the leaves, of finite growth, originated from weak overtopped 

 branches or branch systems of the thallus. Axis and leaf would thus be mor- 

 phologically equivalent organs of common origin. Kidston and Langs afore- 

 mentioned studies of the psilophytes profoundly influenced the fundamental 

 conceptions of shoot, axis, leaf, and sporophyll in vascular plants. The plant 

 body in general now appears to be a simply or complexly branched axis; the so- 

 called fundamental organs are merely parts of this system, specialized for vari- 

 ous functions. Their discoveries induced Zimmermann (1930) to coordinate the 

 new morphological ideas into the telome theory, which became of great interest 

 in connection with the problem of the early evolution of the gymnosperms. 



AVorsdell (1902b) summarized the theories of the nature of the ovular in- 

 teguments. According to the foliar-appendage theory, they are foliar append- 

 ages of the axial nucellus. The sui generis theory regards the integuments as 

 outgrowths of the sporangium, an organ sui generis. According to the foliolar 

 theory, they are the morphological homologues of a three-lobed segment of the 

 megasporophyll (carpel). On the basis of the seed structure in cycads and 

 Cycadofilices, Benson (190-4) then advanced the theory that the seed is a synan- 

 gium, in which the peripheral sporangia have been sterilized and specialized 

 as an integument enveloping the single fertile sporangium. De Haan (1920) 

 reviewed the whole subject, and suggested that in gymnosperms the integument 

 is formed by collateral fusion of a varying number of equivalent elements. In 

 1927 Thomson discussed the evolution of the seed habit in plants on the basis 

 of the sizes of megaspores and microspores at a stage when they have not yet 

 been enlarged to accommodate the prothallium. He found that "heterangy in 

 combination with homospory and heterothally forms the distinctive features of 

 the seed habit, while heterospory represents the culmination of the free-sporing 

 habit." 



AVith a view to the phylogenetic relations, anatomy in general — and especi- 

 ally the vascular system — was more intensely investigated than before. Atten- 

 tion was paid to the structure of both the adult and seedling stages. New dis- 

 coveries made the importance of paleobotany increasingly clear, and it was real- 

 ized that the results of comparative anatomical research on living and fossil 

 forms must play an important role in any scheme advanced. Since Hofmeister, 



