CHAPTER III 
THE FIRST LAND PLANTS 
ALTHOUGH the algz 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- 
alge 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. 
42 
