INTRODUCTION 5 



must move about in order to find their food. Hence, as a general rule, we 

 find that animals have the characteristics of irritability and movement developed 

 to a high degree, whereas plants are stationary and show only slight or 

 sluggish movements in response to stimuli. There are certain exceptions 

 to this general rule. Microscopic plants are often as highly mobile as micro- 

 scopic animals, while, on the other hand, some animals, such as sponges, 

 are sedentary like plants. 



The plant-like tvpe of nutrition is called holophytic and the animal-like 

 type is called holozoic. 



The holophvtic mode of nutrition produces large quantities of the 

 carbohvdrate substance cellulose. This contributes largely to the rigidity 

 of the plant framework, which is not inconsistent with a stationary mode of 

 life, though it would be a serious hindrance to an animal. Cellulose is not 

 formed bv anv animal and is therefore a distinctive mark of vegetable nature. 



Primitive animals are very much like primitive plants : indeed, there 

 are some organisms which can be put into either category, since they are able 

 to feed either holozoicallv or holophytically. For example, Eiiglena is a 

 minute, motile organism, which, under suitable conditions, is able to build 

 up its protein from simple substances in the water around it, like a plant ; 

 but should conditions be unfavourable, it can live as an animal, using protein 

 for its food. 



The Plant Kingdom 



Having thus indicated the chief distinctions between plants and animals, 

 we must now turn our attention to the diiTerent kinds of organisms which 

 belong to the plant kingdom. 



For convenience, plants are classified into phyla, of which the following 

 are the most important : — 



1. Thallophyta (Algae, Fungi and Bacteria). 



2. Charophyta (Stone worts). 



3. Bryophyta (Mosses and Liversvorts). 



4. Pteridophyta (Ferns and Horsetails). 



5. Spermatophyta (Conifers and Flowering Plants). 



The first four phyla are collectively called Cryptogams, because their 

 reproductive organs were for long unknown, as opposed to the Sperma- 

 tophyta or Phanerogams, in which they are easily recognizable in the 

 cones or flowers. Each of these great groups is further subdivided into 

 Orders and Families (see Chapter HI). 



Perhaps the earliest plants, like the earliest animals, lived in water at a 

 time when the dry land was uninhabited. The evolutionary tendency of 

 plants has been, firstly, to migrate from water to land ; and secondly, to 

 make the modifications necessitated by the change from an aquatic to a 

 terrestrial habitat. The Algae, which include the seaweeds, are forms which 

 live almost exclusivelv in water ; hence there is little necessity to develop a 

 complex body, since food substances can be obtained directly from the 



