CLASSIFICATION OF PLANTS BY EVOLUTION. 



139 



So long ago as 1835 Meyer noticed that the distribution of Mono- 

 cotyledons over the globe is regulated, not by temperature, but by 

 humidity ; e.g. in Alabama swamps there are 139 species of Mono- 

 cotyledons, but only 77 of Dicotyledons. 



One of the most characteristic differences between these two great 

 classes of flowering plants is that the embryo of Exogens has two 

 cotyledons, that of Endogens having only one ; but there are several 

 plants among the former class, the embryos of which have only one, 

 while some of the latter have at least a rudiment of a second. Thus 

 Trapa nutans, Banunculus Ficaria, and Carum Bulbocastamim are 

 examples on the one hand, and Asparagus and Tamus of Monocotyledons 

 illustrate the other. 



The first-named is an aquatic plant ; the ancestors of the second 

 undoubtedly icere such ; but the third cannot be noio grouped with it. 

 Though the cause of a monocotyledonous embryo in Monocotyledon is 

 in this paper attributed to degeneration brought about by water, yet as 

 nature has the power to bring about like results from very different 

 causes, it would be rash to say that the single cotyledon of an embyro of 

 Cyclamen, for example, was due to a watery medium. When, however, 

 we find that the development of the embryo of Carum Bulbocastamim is 

 precisely like that of Sparganium ramosum,* and that this character 

 is, as will be seen, associated with so many others to be also found in 

 aquatic Dicotyledons, the presumption is in favour of my contention. 



Following the development of a seed, the next point is the fact that 

 the primary or tap-root is always arrested in Monocotyledons, though it 

 may be temporarily present as in Maize and the Date ; such, however, 

 soon gives place to a succession of adventitious roots issuing from the 

 stems in ascending series. 



Such is also the case with aquatic Dicotyledons, as the Water-crowfoot 

 (Banunculus heterophyllus), Water-lilies, Trapa, many aquatic umbelli- 

 fers, Ceratophyllum, &c. 



Anatomical details follow suit. Thus, while in Dicotyledons the origin 

 of the root-cap is part and parcel with the initial cells at the apex of the 

 root, in Monocotyledons and Nymphceacece the initial cells of the root- 

 cap are distinct from those proper to the root. The large increase in the 

 size of the central vessels is also a common feature in both. 



The rhizome of Water-lilies was at one time supposed to indicate 

 an affinity with Monocotyledons ; but other Dicotyledons also agree with 

 it in the general " dislocation " of the vascular cords so characteristic of 

 the former class. Nelumbium with regular sub-concentric series of cords 

 seems to indicate the transition.! 



The next important point is the leaf arrangements prevailing in 

 Endogens, viz. the " distichous " or two-ranked, as of Grasses and many 

 Orchids, Gladiolus &c. ; and the " tristichous " or three-ranked, as of 

 Sedges ; while the latter leads us to the ternary arrangement so universal 

 in the flowers. 



I have elsewhere shown how the pentamerous arrangement pre railing 

 in the flowers of Dicotyledons arises from opposite leaves, through the 



* Compare the figures by Hegelrnaier, Bot. Zeit. Taf. vii. figs. 28-41. 

 f De Bary's Comp. Anat. fig. 112, p. 255. 



