372 BOTANY OF THE LIVING PLANT 



In physiological resource the Fungi are the most diverse. But each of these 

 includes simple types, which link up more easily with other organisms than do 

 the extremes. Each phylum has worked out its own divergent line of advance 

 independently of the rest. Some degree of parallelism in the progressions 

 may then be anticipated, and is actually found to exist. 



The readiest basis of distinction of these natural groups of Thallo- 

 phyta is by colour. The most important cleavage is according 

 to the presence or absence of chlorophyll, or of some of its deriva- 

 tives. This separates the FUNGI, which have no chlorophyll or 

 kindred colouring matters, from the ALGAE, which have. The 

 Algae again fall into distinct groups on the basis of colour-difference. 

 Those which have full green chlorophyll, such as is seen in 

 Land-Vegetation, are designated the Chlorophyceae. Others are 

 characterised by their olive green or brown colour, which is due to 

 the presence of a brown colouring matter (phycophaein) in addition 

 to the chlorophyll. It is characteristic of the Brown Tangles, or 

 Phaeophyceae. A third series have a prevalent red pigment (phycoery- 

 thrin) in addition to their chlorophyll, and they are called the 

 Rhodophyceae, or Red Seaweeds. These colour-distinctions are not 

 absolutely constant. But it happens that the colours mentioned 

 run closely parallel with other characteristics of form and propagative 

 method, by which these large groups are more strictly defined, so 

 that it becomes a true indication of their distinctness. 



The colourings have a physiological meaning. The absence of chlorophyll 

 indicates dependent nutrition, as in the Fungi. It may be objected that, 

 while the Thallophytes are classified by colour, in Flowering Plants such 

 differences were not taken into account in their classification. But in dealing 

 with Organic Nature, which has progressed along individual lines, consistency 

 of method in classification is not possible, if the grouping is to follow the course 

 which evolution has actually taken. The reason why the method adopted 

 for Flowering Plants will not apply for the Thallophytes is that in the former 

 the change to irregular nutrition happened late. They are plainly Flowering 

 Plants that have changed their mode of nutrition. But in the case of the 

 Fungi we are dealing with a very ancient change. Fungi existed in the 

 Palaeozoic Period. Thus their irregular nutrition will have influenced their 

 development from very early times. 



The colours distinctive of the three groups of Algae are related to photo- 

 synthesis. A brown or red tint makes self -nutrition possible deep down in sea- 

 water. Speaking generally, the Red Seaweeds are prevalent at the lower levels, 

 while the Browns extend from the highest levels downwards, but stop short 

 of the greater depths. The Greens are more widely diffused, but they occur 

 mostly at the higher levels, and they are the prevalent Algae of fresh water. 



If we accept this general view of the Thallophytes, it becomes 

 a question whether there is any living group of organisms 



