+----+------------------------------------+-----------+--------------+ | | Group. | Colour. | Pigments. | +----+------------------+-----------------+-----------+--------------+ | I.|_Flagellatæ._ |Euglenaceæ. |Green. |_Chlorophyll._| | | |Cryptomonadineæ. | | | | | | | | | | II.|_Algæ_-- | | | | | | 1. Myxophyceæ. |Mostly filamen- |Blue-green |Phycocyanin. | | | |tous, chiefly |to violet |_Chlorophyll._| | | |Oscillatoriaceæ |or brown. |Carotin. | | | |and Nostocaceæ. | | | | | 2. Bacillariaceæ.|Mostly pennate, |Golden- |Carotin. | | | |chiefly Navi- |brown. |Xanthophyll. | | | |culoideæ. | |_Chlorophyll._| | | 3. Chlorophyceæ. | (i) Protococ- |Green. |_Chlorophyll._| | | | cales, Ulo- | | | | | | trichales, | | | | | | Conjugatæ, | | | | | | etc. | | | | | |(ii) Heterokontæ.|Yellow- |_Chlorophyll._| | | | |green. |Xanthophyll. | | | | | | | |III.|_Bryophyta._ |Filamentous moss |Green. |_Chlorophyll._| | | |protonema. | | | +----+------------------+-----------------+-----------+--------------+ The importance of the lower algæ from a biological standpoint has long been recognised, since their extremely primitive organisation, coupled with their ability to synthesise organic compounds from simple inorganic substances, singles them out as being not very distantly removed from the group of organisms in which life originated upon the earth. But the possibility of their having a very much wider economic significance was completely overlooked until about a quarter of a century ago, when Hensen demonstrated their importance in marine plankton as the producers of the organic substance upon which the whole of the animal life of the ocean is ultimately dependent. In consequence, it has been generally assumed that the growth of algæ, since they contain chlorophyll, is entirely dependent on the action of light. Hence the recent idea of the existence of algæ which actually inhabit the soil has been received with a certain amount of scepticism, though the results of modern physiological research on a number of the lower algæ show that there is very good reason to believe that such a soil flora is entirely possible.