INTRODUCTION 



usually the first to be seen. Since green plants alone can manufacture 

 these carbohydrates, the whole carbon supply for both plants and 

 animals is ultimately dependent upon these green plants. 



While the power to assimilate carbon-dioxide seems to be confined 

 to green plants, it is not impossible that certain Bacteria which do 

 not possess chlorophyll, may have this power to a limited extent. 

 In such forms there is found a red or purple pigment which may pos- 

 sibly replace chlorophyll in the process of decomposing carbon-diox- 

 ide. Moreover, the so-called nitrifying Bacteria are able to decompose 

 the simple nitrogen compounds, like ammonia, and manufacture the 

 nitrogen compounds which are available for the higher plants. Still 

 other Bacteria, which inhabit tubercles on the roots of various legu- 

 minous plants, can use free nitrogen. The fixation of nitrogen by 

 these Bacteria is of very great importance in the economy of nature, 

 but has only been understood of recent years. 



With the possible exception of a few Bacteria, all plants without 

 chlorophyll, such as Fungi and many parasites and saprophytes 

 among the flowering plants, e.g. Dodder, Indian-pipe, etc., must 

 obtain their carbon in the form of organic compounds, thus behaving 

 like animals. In case they attack living plants or animals, as do 

 many Bacteria and Fungi, or such flowering plants as Dodder or 

 Mistletoe, they are called parasites; if they feed on dead matter, like 

 many Moulds, Toadstools, etc., they are saprophytes. Thus the 

 power to manufacture the primary organic compounds is by no means 

 universal among plants, and cannot be used as a certain criterion to 

 distinguish them from animals. 



Structural Resemblances in Plants and Animals. The essential struc- 

 tures of plants and animals are extraordinarily similar, so great, 

 indeed, that among the simpler forms it is often difficult to say to 

 which kingdom they belong. In all cases, life is bound up with the 

 presence of protoplasm, which so far as can be judged by ordinary 

 physical and chemical tests is alike in plants and animals. Of 

 course there must be inherent peculiarities in the protoplasm of dif- 

 ferent organisms, but at present we have no means of distinguishing 

 these. The simplest known organism consists of a minute, usually 

 nucleated mass of protoplasm which exhibits sensitiveness, motility, 

 and the power of nutrition and respiration. By simple division two 

 new individuals arise the simplest form of reproduction. In short, 

 such a nucleated particle of protoplasm is capable of manifesting all 

 the characteristics of a living organism. 



Multicellular Organisms. While many animals and plants consist 

 of a single nucleated protoplasmic mass, or are " unicellular," much 

 the greater number are composed of cell-aggregates or tissues, but 

 each individual, however complicated, may be traced back to a single 

 such cell. The extraordinary likeness in the structure and behavior of 



