Till': sr\(;|.: si:rnN(; o., 



Towins with a phnikton i.cl (a llun-mcshcd uv\ cf l,oltii,K ,-l„tl,) 

 near the surlacc of tlu> ocean on an early summer day would yield 

 a very different distribution of organisms from those collected "on ;, 

 fall or winter day. There is a seasonal variation in distrihutioii. 

 The eggs and larvae of animals ar(> abundant in the spring and early 

 summer, while great numbers of algae appear then which are Hot 

 found later. This rhythm of plant life is believed to be correlated 

 with a turn-over of the available phosphates and nitrates in the water. 

 In the winter, the (^ooler top layer of water sinks and pushes up the 

 water rich in the salts necessary for plant growth from underneath, 

 so that with the coming of warmer weather tlu^ life cycle goes on and 

 a seasonal rhythm of algae appears. This turnover of plant and 

 animal life is very great. The fishing industry on the Grand Banks 

 and in the North Sea is largely due to the occurrence of this great 

 seasonal rhythm of plankton. 



There is also a considerable variation in the numbers of plankton 

 near the surface of the water during the day and night. Many crus- 

 taceans, for example, come to the surface at night and go down in 

 the daytime, while green algae are usually nearer the surface during 

 the day. 



In oceans and lakes, there is a more or less distinct zoning of living 

 forms, depending on the depth of water, the type of shore, or the kind 

 of bottom. A very different fauna and flora exist on a rocky coast 

 from that along a sandy beach. The forms of botii plants and ani- 

 mals are different in salt and fresh water areas. 



Life in the Air 



Here life is more circumscribed. There are no true air plants unless 

 they be the so-called epiphytes of the tropical rain forest, some algae, 

 such as the Pleurococcus found on the bark of trees, or the lichens, 

 which encrust rocks and tree trunks. The reproducti\'e bodies of 

 plants, such as spores, seeds, and fruits, are furnished with adai)ta- 

 tions which enable them to pass long distances through the air, thus 

 allowing new areas to be populated. In animals where locomotion 

 is possible various special adaptations exist. Flying animals hiivv 

 their wings placed wdiere they will not onlv cause the liody to mo\e 

 forward, but also assist in balancing it. Instead of one ])ropeller 

 placed astern, as in fish, flying animals have two paired i)roi)ellers 

 placed forward at a greater breadth of beam. The body is not onl.\- 

 streamlined, but in higher forms special adaptations exist for protec- 

 H. w. H. — 3 



