242 
THALLOPHYTES. 
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 of 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. Algoe, 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 pecuharities of their organisation, to live in water or in damp places. Their 
dependence on assimilation requires that Algse should inhabit localities where there 
is free access of fight, 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 aff"ects 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 sexuafity 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 fittle 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. 
