200 ELEMENTS OF SCIENCE 



which give out oxygen, when feeding, by absorbing the 

 carbon of the atmospheric carbonic acid. 



All the higher plants resemble, in this respect, the 

 green unicellular ones, and therefore enormous volumes 

 of oxygen are given forth by the vast forests and exten- 

 sive grassy plains which clothe the earth's surface, as 

 also by the masses of seaweed in the ocean. 



There is a sea-weed called Lessonia which forms sub- 

 marine forests, with stems like the trunks of ordinary 

 trees, while the sea-weed called Macrocystis may attain a 

 length of 700 feet. The group of small sea-plants 

 known as Floridice are amongst the most delicate and 

 elegant of vegetable structures. 



p JG ..,. Animals, like fungi, cannot dissolve car- 



bonic acid, absorb its carbon and let its 

 oxygen go free; but in breathing they all 

 give forth large quantities of carbonic acid, 

 at the same time absorbing a great amount 

 PROTOCOCCUS f t ^ ie ox ygen given forth by green plants. 



with two This is the reason why animals soon become 



Vibmtile suffocated when enclosed in anything which 



Cilia. denies them a supply of fresh air. When so 

 enclosed, they soon exhaust the supply of oxygen avail- 

 able, and die of suffocation, because they can then no 

 longer exchange their carbonic acid for it. 



Some of the lower plants move about in water with 

 much activity, as for example, does the alga called Proto- 

 coccus (Fig. 31), which moves by means of two minute hair- 

 like processes termed vibratile cilia, which effect repeated 

 lashings, the cause of which is, as yet, quite unexplained. 

 Another very simple plant is called Volvox (Fig. 32), and 

 consists of a spheroidal aggregation of cells bearing 

 outwardly a multitude of projecting cilia, the regular 

 lashings of which cause the spheroidal whole to rotate, 



