30 AN INTRODUCTION TO THE STUDY OF FOSSILS 



they absorb the food directly from the host, either plant or 

 animal, which has manufactured it. 



In respiration plants take in air through small openings 

 (stomata) which are especially abundant upon the leaves. The 

 use made of the oxygen is shown in the rise of temperature and 

 in the energy of growth. Since respiration is in plants as well 

 as in animals a breaking down process, a waste product, — 

 carbon dioxid, is thrown out. 



Reproduction among plants may be either sexual or asexual. 

 Asexual plants reproduce either by simple division, as among 

 the bacteria, or by merely detaching single cells from themselves, 

 as among certain algae ; each of these detached cells, called in 

 the latter case a spore, is capable of developing without fer- 

 tilization into an independent plant. In sexual reproduction 

 the plant likewise separates from itself certain single cells, but 

 these cells lack the power of developing by themselves ; be- 

 fore growth can occur each must unite with another cell, either 

 from a separate plant or from different parts of the same plant. 

 The larger of these complementary reproductive cells in the 

 higher plants contains the egg and is the female ; the other 

 cell, which may or may not be free-swimming, contains the male 

 element. In all except the seed-plants the presence of external 

 fluid water is a necessity, otherwise the male cell cannot travel 

 to the female and produce fertihzation. 



Plants as indicators of climate. — Plants, at least the later 

 ones, are excellent criteria of climatic conditions ; their ina- 

 bility to migrate under the stimulus of the annual alternation 

 of cold and warm, wet and dry seasons makes them more 

 valuable than animals as geologic thermometers. But that in 

 the Mesozoic, and especially in the Paleozoic, geographic dis- 

 tribution was as sharply limited by climatic environment as 

 now is unlikely. Some persistent primitive types such as the 

 common brake (Pteris) which thrive in both temperate and 

 tropical regions are suggestive in this connection. The world- 

 wide distribution of the ancient floras was mainlv due to their 



