J5 6 BOTANY OF THE LIVING PLANT 



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 apparently 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. The seed-bearing parasites 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 Palae- 

 ozoic Period. Thus their irregular nutrition will have influenced their 

 development from very early times. 



The colourings have a physiological meaning. The absence of chlorophyll 

 indicates dependent nutrition, as in the Fungi. The colours distinctive of 

 the three groups of Algae are related to photosynthesis. 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 which 

 represents approximately a source from which they may have 

 originated. It is a very general opinion that such a source is to 

 be found among the Flagellatae, a family which it is difficult to 

 refer definitely either to the Kingdom of Animals or of Plants. It 

 includes many of those organisms which cause certain diseases in 

 man and other animals, and these are more definitely animal in 

 their characters. But others, such as Euglena, possess features 

 characteristic rather of Plants. Euglena is found commonly in 

 summer, colouring the foul water that drains from manure heaps 

 a bright green. The organism is then seen in the motile state, as a 

 free-swimming, naked protoplast of elongated form, propelled by 

 a single flagellum (Fig. 265). There is a central nucleus (n), several 

 green-coloured chromatophores which vary according to the con- 

 ditions (ch), a contractile vacuole (v), communicating by a canal 

 or funnel with the exterior, and a red eye-spot or stigma lying at 

 the junction of canal and vacuole. The flagellum passes down- 

 wards through the canal, and is attached by a branched base to 

 the inner surface of the vacuole. In this state Euglena can feed 

 itself by photosynthesis, but it probably obtains simultaneously 

 some degree of saprophytic supply from the foul water in which it 

 lives. When well nourished it may contain large paramylon bodies, 

 but not starch. It multiplies by fission, the nucleus dividing first 



