184 SECTIONAL ADDRESSES. 



reduction-series can be recognised in some groups of the Algae, there is no 

 evidence at all to indicate that freshwater Algae as a whole have undergone 

 reduction. It must be remembered too that a large number of similar 

 unspecialised forms are found in the sea. Moreover, with the facts of 

 parallel development before one the supposition of a general reduction 

 becomes practically untenable. We have no knowledge of any forms 

 from which the filamentous Heterokontae or Chrysophyceae, for instance, 

 could be derived by reduction, for the relationship between Chrysophyceae 

 and Brown Seaweeds formerly entertained is probably fallacious and now 

 no longer credited by most authorities. Until some real evidence can be 

 adduced that reduction has occurred, it appears more logical to regard 

 the filamentous forms in the different classes as the end-points of an 

 upgrade development. Further facts that lend support to such an inter- 

 pretation are the wide distribution of the simpler, less specialised, members 

 of each class (very noticeable in the case of many groups of the Isokoutae), 

 while the more highly specialised forms are commonly of more restricted 

 distribution. Within the Isokontae too anisogamy or oogamy are asso- 

 ciated with the advanced forms and are mainly a feature of the specialised 

 filamentous types, a fact which supports the idea of a progression, rather 

 than of a retrogression. In other classes also sexuality is usually found 

 only in those forms which have the more elaborate organisation. 



Among the numerous septate filamentous Isokontae, it is possible 

 to distinguish four separate series, of which the Oedogoniales and Con- 

 jugatae have already been recognised as specialised along directions of 

 their own. Of the other two, the Ulotrichales are the simpler and the 

 Chaetophorales the more complex, both possibly originating from a common 

 stock. Many authorities, in fact, fail to distinguish these two groups, 

 but the organisation of the Chaetophorales is so distinct from that of the 

 Ulotrichales that, from the standpoint of comparative morphology, 

 their separation is desirable. Whereas in the Ulotrichales we have a 

 simple or branched filament attached by a more or less elaborate basal 

 cell, the central types among Chaetophorales are distinguished by the 

 possession of a plant-body showing differentiation into a prostrate system 

 of creeping threads serving inter alia for attachment to the substratum 

 and a projecting system which is more or less richly branched. This 

 differentiation is seen for instance in many species of Stigeoclonium, 

 Coleochaete, and Trentepohlia, representing three distinct families and 

 three distinct developmental lines within this group. Among its numerous 

 members there is much variation in the relative differentiation of the 

 creeping and projecting systems, and reduction of the latter has led to a 

 whole series of specialised prostrate or discoid types (Aphanochaete, 

 Protoderma, &c). It is to be noted, too, that the Chaetophorales exhibit 

 a greater morphological diversity and a capacity for existence under 

 more varied circumstances than any other group of Isokontae ; and in the 

 Trentepohliaceae include one of the most vigorous and highly differentiated 

 families of terrestrial Algae. 



The type of construction just considered is not encountered in any of 

 the other nine classes of Protophyta previously mentioned, although 

 hinted at in the Chrysophyceae and some Myxophyceae. An analogous 

 differentiation of the filamentous thallus into a creeping and a projecting 



