248 ELEMENTARY SCIENCE 



see one of them with the naked eye. Such plants still live. 

 That green coating you sometimes see on bark, or where 

 water drips on rock, is composed of millions of such one- 

 celled plants (see Fig. 89). 



Nutrition. Think of the life-problem of such a plant. 

 There it floats near the surface of the water on a bright, 

 sunshiny day. To keep alive it needs sunlight and air and 

 water and some of the substances dissolved in the water. 

 All these things are abundant where it lives, and one might 

 think that the little plant would have an easy time. This 

 is true as long as summer lasts, but if this little plant lives 

 where water freezes it must change its form to keep alive. 

 This it may do by forming a thicker coat around itself, 

 and sinking to the bottom. There it may rest for a long 

 time and still keep alive (just as seeds keep alive) in the 

 coldest weather. When spring comes it can come to the 

 surface and start its active life again. 



Just keeping alive is not all that there is to plant life. 

 If it were, then the plants on which we live would never 

 have come into existence, and we should never have come 

 into existence. All through their history plants seem to 

 have done everything in their power to spread, to spread 

 everywhere that plants can live. There seems to be no 

 place in the world where plants can grow but that, sooner 

 or later, they find that place and grow there. This means 

 that they must reproduce. It also means that in the long 

 history of plants many different kinds have appeared, 

 and so we find different kinds of plants in different kinds 

 of places. We do not understand how the different kinds 

 of plants have appeared, but we do understand that they 

 all have come from the same ancient ancestors, and that 



