76 ELEMENTS OF BIOLOGY 



REPRODUCTION, which always involves components from two organ- 

 isms as a preliminary to reproduction. In the blood of Man the para- 

 site reproduces by rapid divisions of the cell, a process that is 

 independent of any other individual cell. This is asexual reproduc- 

 tion. In the life history of the malarial parasite these two processes 

 are in necessary sequence; the sexual always follows the asexual, and 

 the mosquito host must always follow the human host. The disease 

 is therefore not transmissible from one human to another, and its 

 control involves the eradication of the mosquito. 



Although all classes of Protozoa contain some kinds that are 

 parasitic, not all the animal parasites that infest other animals are 

 Protozoa. Some of the parasites of Man, for instance, are multi- 

 cellular animals that are quite large, for example, the tapeworm 

 (p. io6). Some have very complicated life histories, requiring several 

 hosts, or spending part of their life cycles as free living and part as 

 parasitic animals. The parasitization of Man by unicellular plants 

 will be discussed farther on. 



Enzymes. Simple plants most clearly illustrate certain bio- 

 logical principles that it is now desired to develop. It has been 

 stated that the Plant kingdom, like the Animal kingdom, may be 

 divided into sub-kingdoms, the single cell or simple plants, Proto- 

 phyta, and the Metaphyta, or many-cell plants. In the sub-kingdom 

 Protophyta a single phylum is recognized, the phylum Thallophyta. 

 The Metaphyta are three phyla: the Bryophyta including the mosses 

 and their allies, the Pteridophyta, the ferns and their allies, and 

 the Spermatophyta or seed-bearing plants. Of the phylum Thallo- 

 phyta, there are three series: the Algae, which include the green 

 pond scums; the Fungi, which include the Yeasts and Molds; 

 and the Schizophyta, which are the plants which reproduce by 

 binary fission and include the Bacteria (see top of page 77). 



The A\gx contain chlorophyll, or some substance closely related, 

 and are holophytic. The Fungi and Schizophyta are saprophytic. 



