446 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



represent completely independent phyla which exhibit striking parallelisms and 

 convergences ? 



2. Do the monocots or the dicots — the existence of an affinity being granted 

 —represent the more primitive group, and monocotyly or dicotyly the ancestral 

 condition 1 



3. Whatever their origin, are the monocotyledons a natural group or an 

 artificial aggregation? 



4. If they did have a single source, from what group were the monocots de- 

 rived and under what conditions ? 



5. What living monocotyledons retain the greatest array of primitive features ? 

 So great is the majority of those who have recognized a genetic relationship 



between monocots and dicots that we may properly note here primarily a few 

 dissenting opinions. Lindinger (1910), stressing the loss of the primary root 

 with the substitution of an adventitious system and the absence of vessels in 

 the secondary xylem, concluded that monocotyledons have had a quite discrete 

 origin. In the belief that flowering plants arose from several or many Mesozoic 

 "Protangiosperms," some dicotylous, some monocotylous, separate origins for at 

 least some groups of the latter have been proposed by several authors (Engler; 

 Campbell, 1930b; Pulle, 1938; Schaffner). A separate descent from different 

 members of the Bennettitales was urged by Calestani and Wodehouse (1936a); 

 the latter even thought Nymphaeaceae might represent an independent line 

 from the same source. Von Tuszon (1936) preferred separate but similar ori- 

 gins from Gnetales or Gnetales-like ancestors. A derivation of monocotyledons 

 from Lycopodiinae and of dicotyledons from other vascular crytogams was pro- 

 posed by Appl (1937, 1937-1938), who has advanced several remarkable but 

 quite unsubstantiated speculations; Conzatti (1942) saw in Isoetes a possible 

 forerunner of Gramineae. Bertrand (1947b) would trace both groups — like all 

 other major lines in tracheophytes — independently from unicellular green algae. 

 In accordance with his stress on the importance of "stachyspory" versus "phyllo- 

 spory," Lam regarded these conditions as outweighing the distinction between 

 monocotyly and dicotyly; Pandanaceae, at least, he regarded as "stachysporous," 

 the great bulk of monocotyledons as "phyllosporous," Liehr (1916) resorted to 

 cytological investigation to settle the question of affinity between the great angio- 

 sperm classes, but was unable to reach any firm decision, perhaps because he con- 

 fined his observations to three species from eacli of the two groups. The contrary 

 view, that monocots and dicots actually differ very little from each other, has 

 been expressed repeatedly (Bessey, Sargant, Arber and Parkin; Worsdell, 1908; 

 Coulter and Land, 1914; Coulter, 1915; Parkin, Zinke, 1924; Gliick, 1925; Camp- 

 bell, Schnarf, Maheshwari; Metcalfe, 1946; Johansen). Indeed, Suessenguth 

 (1921) went so far as to suggest that monocots are in reality only a conventional 

 group "like the Sympetalae," and that several lines stemming from dicots may 

 have reached a "monocotyledonous stage." 



In the nineteenth century, monocotyledons usually preceded dicotyledons in 

 the sequence of orders, but the pendulum has now swung far in the opposite 

 direction. Of those few who have more recently reaffirmed the greater relative 

 antiquity of monocots, Lindinger, Schaffner, Conzatti, and Corner have empha- 

 sized the retention of a cy cad-like habit by woody monocotyledons. Schellenberg 

 thought that lack of secondary growth, tristichous foliage, and trimerous flowers 



