616 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



vegetable food by animals. But our knowledge in this respect is so fragmentary 

 and uncertain that for the present we cannot make use of these conditions in tin- 

 limitations of the groups. 



The capacity for sexual union is of the utmost importance in defining plant 

 groups. Species which can unite sexually belong undoubtedly to the same group. 

 Nothing can be urged against this principle, and if it could be universally applied, 

 the division of the groups would be settled. But in this matter there are very 

 many pros and cons. The converse of the proposition requires consideration. It 

 will not do to say that all plants which cannot unite sexually belong to different 

 groups. It has been shown that crossings can be successfully effected in Orchids 

 which all Botanists regard as members of different genera, but, on the other hand, 

 it is demonstrated that crossings between very similar species of the Umbellifer 

 family lead to no fruit formation. No one, however, would conclude from this 

 that these Umbellifers belonged to different groups. On reflecting in what a small 

 number of flowering plants the fertilizing process has hitherto been observed, and 

 remembering that the fertilization of many Thallophytes is still totally unknown, 

 the hope of being able to utilize these conditions in limiting the groups becomes 

 very much lessened. 



In the review of the various groups of the vegetable kingdom which follows 1 

 no attempt is made to present the groups in the form of an ideal natural system. 

 So far as the Thallophytes, Bryophytes, Pteridophytes, and Gymnosperms are con- 

 cerned, there is a very general consensus of opinion amongst Botanists, and the 

 serial arrangement here followed is in harmony with it. But as regards the Angio- 

 spermous flowering plants, and in particular the Dicotyledons, it is as yet too early 

 in a book of this nature to embody all the most recent suggestions as to the affinities 

 of the various families. Attention w r as drawn on p. 605 to the system of Alexander 

 Braun, and it was pointed out that he was the first to try and break up the large 

 and unsatisfactory class Monochlamydeas or Apetalae, and to relegate its families in 

 part to their true position. This attempt has been very fully carried out by Eichler 

 (1883), and by Engler (1892): these two Botanists admitting only two classes of 

 Dicotyledons (Choripetalfe or Archichlamydea? and Sympetalae). But as yet many of 

 their placings of individual families are but tentative, and we may well wait a few- 

 years for a system on these lines to settle down into more or less permanent form. An 

 instance of too hasty rearrangement of a natural system to meet recently discovered 

 facts may be quoted here. In 1891 Treub discovered that Casuarina possessed chala- 

 zogamic fertilization, and in 1892 Engler (following Treub) separated Casuarina £ri 'in 

 all other Angiosperms as the sole genus in a new class Chalazogamse. Since then 

 it has been found (see p. 413) that chalazogamic fertilization is much more general 

 than was at first supposed, and that in the group Amentaceae it is widely spread, 

 though by no means of universal occurrence. To break up the Amentacese in the 

 drastic manner involved, if the class Chalazogamse be maintained, seems a most 



1 Cf. editorial note at commencement of this volume. 



