6 A TEXTBOOK OF THEORETICAL BOTANY 



water in which they Hve. The simpler members consist of a single cell, but 

 in more advanced forms the cells are joined together in groups or in filaments 

 in which every cell retains its individuality and is identical with every other 

 cell. It is only among the more specialized Algae that we find a division 

 of labour and a marked morphological differentiation of the cells, but even 

 then there is nothing approaching the development of the stem, root and 

 leaf of the flowering plant. So long as plants remain supported on all sides 

 by water, their bodies need little or no reinforcing to keep them erect, but 

 in even the simplest land plants some kind of supporting tissue is found to 

 be present. This usually takes the form of hard material, called wood or 

 xvlem. In the Bryophyta, which though terrestrial are small plants, e.g.. 

 Mosses, supporting tissue is poorly developed and does not consist of wood. 



In the Pteridophyta, particularly in the Filicales or Ferns, very large 

 leaves may be developed in association with which a particularly complex 

 woody tissue has arisen. 



The Phanerogams have developed both the tree and herbaceous types, 

 but the wood system is more complex than in the ferns. At the present 

 day the herbaceous plant is probably the most successful type of vegetation. 



This, then, very briefly is the conception of the range of organisms which 

 are included under the term plants. Many are very different indeed from 

 the wayside herb, or the garden flower with which we are familiar, but they 

 all agree in having one common feature, namely, that they live holophytically. 



The essential feature of holophytic nutrition is that food is absorbed 

 entirely in solution, never in solid form. Where the food solution contains 

 organic substances derived from the decay of other organisms the plant is 

 called a saprophyte. Where the food is derived from the living body of 

 another organism the plant is called a parasite. Such plants are only a 

 minority. Green plants, on the other hand, take up only inorganic substances 

 from which they build up their own organic material. They are said to be 

 autotrophic, which means " self- feeding." This mode of nutrition is shown 

 by no organisms except plants, and we may briefly refer to the process here. 



First of all the green plant combines the free carbon dioxide of the air 

 with water from the soil, to form carbohydrates. Every green plant obtains 

 its carbohydrates by means of this process of feeding, which is called carbon 

 assimilation or photosynthesis. This process can take place only in the 

 presence of light and chlorophyll. 



Chlorophyll is a green pigment of a complex nature, containing carbon, 

 oxygen, hydrogen, nitrogen and magnesium. It is found in all the green 

 parts of plants, particularly in the leaves, where it occurs in specialized 

 protoplasmic structures which are known as chloroplasts . Only in cells 

 containing chloroplasts can the photosynthetic production of carbohydrates 

 take place. The necessary water and some mineral salts in solution are either 

 obtained directly from the water in which the plants live or, in land plants, 

 from the soil through the roots. In the body of the plant certain of these 

 salts containing nitrogen are combined with carbohydrates to form proteins 

 and, finally, protoplasm. 



