COMPARISON OF ANIMAL AND PLANT BIOLOGY 131 



and whether the water moves in a circuit, as in animals, or up 

 and down, as in plants, is a matter of secondary importance. 

 The complete circuit in animals has for them the advantage 

 that a swifter movement of water (of the blood) makes 

 possible more rapid distribution of foods, oxygen, and ex- 

 cretions. But plants, because of the sluggish life-activities 

 of their cells, can get along with a less rapid distribution of 

 the necessary foods and oxygen, and excretions. 



We see, then, that while sap in our common plants and 

 blood in animals appear at first sight to be widely different, 

 careful study of their respective functions shows that they 

 are really doing similar work in connection with the life- 

 activities of protoplasm. 



122. Reproduction. This process has many similarities 

 in animals and plants. The frog and the bean plant are 

 typical of the great majority of living things in that they 

 reproduce by means of two cells which unite to form a ferti- 

 lized egg-cell able to develop into a new organism. In both 

 animals and plants the egg-cells are very similar in their 

 microscopic structure, and in each case fertilization causes 

 the egg-cell to begin division into a number of cells which 

 form the embryo. The cell from the pollen-tube which fer- 

 tilizes the egg-cell in flowering plants ( 75) corresponds 

 to the sperm-cell of animals ( 57). In some flowerless 

 plants (e.g., sea- weeds) there are motile sperm-cells which 

 swim in water just as those of fishes or frogs do ; but these 

 plants live where water surrounds the ovaries producing egg- 

 cells and the spermaries producing sperm-cells, and so the 

 sperm-cells can readily swim to the egg-cells and fertilize 

 them. The absence of water surrounding the flowers of the 

 higher plants evidently makes such a method of sperm-cells 

 reaching egg-cells impossible; and so the method of com- 

 munication by means of pollen-tubes has been developed as 

 an adaptation to the peculiar dry conditions under which 

 fertilization must take place in most flowers. , However, it 



