242 THALLOPHVTES. 



attaches the least importance to the fact that the want of chlorophyll and the pe-^ 

 culiar mode of life of these plants gives them so different an appearance from 

 that of their nearest allies. It is one of the most beautiful results of a truly 

 scientific morphology and classification that, among Phanerogams, the remarkable 

 habit of parasites and saprophytes is regarded as an altogether secondary matter. 

 But the same principle should also be applied in determining the systematic 

 relationships of Thallophytes ; — habit and mode of life, the presence or absence 

 of chlorophyll should also be treated as characters of altogether subordinate 

 importance. All Thallophytes which are destitute qf chlorophyll — z'. ^. all those 

 which have hitherto been termed Fungi — must necessarily agree with one another 

 more or less in their habit and mode of life, because they are all adapted to absorb 

 organic carbonaceous nutriment from their environments. If they obtain it from 

 living bodies, we have parasitism developed in its various forms ; if they have 

 the capacity of consuming dead organic remains, the habit and mode of life of 

 the plant must vary accordingly. Algae, in the sense in which the term has hitherto 

 been used, are able themselves to produce carbonaceous food-materials out of 

 carbon dioxide by assimilation; they are not therefore usually either parasites or sa- 

 prophytes, but can maintain an independent life; they are howe\er compelled, by 

 the peculiarities of their organisation, to live in water or in damp places. Their 

 dependence on assimilation requires that Algae should inhabit localities where there 

 is free access of light, while Fungi are not absolutely dependent on light for 

 their supply of food. 



But all these facts are of altogether secondary importance in determining 

 degrees of affinity in the compilation of a natural system of classification of 

 Thallophytes. This object can be attained only by a comparison of such mor- 

 phological characteristics as a thorough knowledge of development reveals. The 

 determining considerations of a morphological nature are in Thallophytes, still 

 more than in other groups of plants, dependent on the question whether they 

 possess sexual organs, and, when this is the case, how these are formed, how the 

 act of fertilisation is effected, and especially what is the nature of that structure 

 which results either directly or indirectly from it, in one word, how the act of 

 fertilisation affects the entire course of development. 



We have already described the more important forms of the organs of 

 fertilisation in Thallophytes, and the origin of an alternation of generations con- 

 sequent upon it. If now the plants which agree in these characters are compared 

 with one another, it is seen that the remaining morphological facts also suggest 

 a close affinity. The structural peculiarities connected with sexuality may there- 

 fore be regarded as the guiding characteristics, by which we are directed to 

 relationships within the group. With our present still very imperfect knowledge of 

 Thallophytes it is however not surprising if, in a classification founded on these 

 principles, forms are nevertheless occasionally found placed near each other which 

 appear to have but little affinity. This is unavoidable, because the intermediate 

 transitional forms are unknown; and it must moreover be observed that in Thal- 

 lophytes of a simple structure the morphological characteristics are more easily 

 concealed by physiological adaptations and by changes in habit than in the higher 

 plants. 



