INTRODUCTION. 
241 
off. In the genus Vmicheria we find all stages of transition from motile cells to 
gonidia which simply fall off. 
The Classification of Thallophytes has been till quite recently based essentially 
on characteristics relating merely to the mode of life, according to which they have 
been divided into three classes, — Algae, Fungi, and Lichens ; the Characese have 
been sometimes included under Algae, sometimes altogether separated from Thallo- 
phytes. But since more accurate investigations have enabled us not only to elucidate 
the morphological significance of the growth of these plants, but also to discover the 
sexual organs in the main divisions, and in many cases to follow the whole course of 
development, this classification can no longer be maintained, since it depends 
essentially only on differences in the external appearance and mode of life, while on 
the other hand it is seen that a totally different classification is necessitated by their 
morphological characteristics. The admirable labours of Schwendener, for example, 
have shown that Lichens, hitherto considered as a separate class, must not only be 
included among Fungi, but must be regarded as a section of a particular order, the 
Ascomycetes. Since we have become more accurately acquainted with the sexual 
organs of the Coleochoetese and Florideoe, it can scarcely be doubted that these Algae 
have a close affinity to Characeas in the structure of their frucdfication. We can 
therefore now distinguish only two classes of Thallophytes, — Algae and Fungi. But 
it has long been admitted that it is impossible to draw any satisfactory boundary-line 
between these two classes ; several writers have indeed frequently pointed out that 
some famihes of Fungi must be closely associated with certain families of Algae. It has 
been recognised more and more clearly that these two classes are separated only 
by a single distinguishing character; if the two old-established groups are to be 
retained, the only distinction between the two (and the one adopted in the earlier 
editions of this book) is to place under the head of Algae all those Thallophytes 
which contain chlorophyll, under the head of Fungi all those which do not. But 
this separation is altogether artificial, and could only be tolerated so long as want of 
an accurate knowledge of the morphology of these plants compelled us to admit 
a classification having no foundation in morphology. In the present state of our 
knowledge, in which at least the morphological foundation for a scientific classi- 
fication of Algae and Fungi may be laid down, it is not only permissible but 
incumbent, in the interest of progress, at least to attempt a morphological classi- 
fication of Thallophytes. 
The first point to note is that the presence or absence of chlorophyll can be 
no sufficient reason for separating plants which are nearly related to one another 
morphologically, and which agree in their structure, their sexual organs, and their 
alternation of generations. In Phanerogams this principle is thoroughly admitted. 
If all Flowering-plants which do not contain chlorophyll were formed into one 
class in contradistinction to those which do contain it, the Raffiesiaceae, Balanopho- 
raceae, Corallorhiza, Ci/scuta, Orobanche^ Monotropa, &c. would have, in spite of 
the differences in their organic structure, to be combined into one class, and 
removed from their true relationship. No one however disputes that Cuscuta 
belongs to the Convolvulaceae, Orohanche to the Labiatiflorae, Monoiropa to the 
Pyrolaceae, and Corallorhiza to the Orchideae. These affinities are inferred, among 
Phanerogams, chiefly from the structure of the flowers and the embryo, and no one 
R 
