CHAPTER III 



THE FIRST LAND PLANTS 



Although the algae prefer for the most part salt or 

 fresh water, many occur on land, and in the most un- 

 expected and peculiar places. 



Sometimes one sees a pebble lying on the seashore 

 with a strange blood-red stain upon it. When this 

 colouring matter is scraped off, it is found to be a 

 minute parasite alga (Petrocelis or Hildenbrandtia). 

 The greenish powder which covers old stems and 

 branches of spruce and other trees consists of another 

 alga (Pleurococcus). One may discover such land- 

 algae high up on boulders of Alpine craigs, on small 

 stones exposed in a lowland pasture or on stone walls. 



Perhaps the most remarkable of them is a moist, 

 unpleasant, dirty green bubble-like jelly which appears 

 after wet weather on gravel walks and stone walls. In 

 dry weather it dries away and becomes almost invisible. 



This is an alga, nostoc ; the cell walls, which are in 

 most plants very narrow, are in this group changed 

 into a jelly-like material * and are inordinately swollen. 

 In consequence the tiny speck of protoplasm within 

 them is very well protected against both vegetable and 

 animal enemies. But we mention it here because of 

 an interesting discovery, which was that such a patch 

 of nostoc is able somehow to obtain nitrogen, not from 



* This gelatinous substance is a modification of cellulose containing pro- 

 portionately more molecules of water. It is also usually formed out of cellulose 

 by water, and may be considered another case of fitting reaction. The slimy 

 surfaces of all submerged plants are probably due to some similar substance. 



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