Ch. V, 10] SUMMARY OP PLANT TISSUES 261 



soil, lessens the continuity of the water films (page 240), 

 and hence checks the free movement of soil water to the 

 surface and its loss there by evaporation. The method is 

 important in dry farming, of which the first problem is the 

 conservation of soil moisture. 



10. SuJIilART OF THE FUNCTIONS AND TISSUES OF PLANTS 



Having considered separately the principal tissues of 

 plants, it will now be well to summarize them together 

 to show their connections and the systems they form. 



The functions of plants are performed by their proto- 

 plasm, which is subdivided into cells. When these cells 

 are speciahzed to a particular function in considerable num- 

 bers, we call them collectively a tissue. Organs are enlarge- 

 ments or extensions of the plant body of such shape and 

 position that they bring the tissues devoted to a function 

 into advantageous relation with some external condition 

 crucially affecting that function, — as witness the leaf in 

 its relations to light. Thus organs do not so much perform 

 functions as permit tissues to perform functions in advanta- 

 geous relation to external conditions essential to their 

 operation. 



While some functions are performed almost wholly by the 

 protoplasm inside the cells, others involve great specializa- 

 tion of the shape, thickness, and other features of the cell 

 walls. 



With respect to the tissues involved, all functions fall 

 into four classes, as follows : 



I. Functions performed in tissues requiring special posi- 

 tions and constituiing morphological systems. 



1. Frotectiox against unfavorable external influences, 

 notabl}' dryness of the air and entrance of parasites. The 

 function requires external position, continuity, and a water- 

 proof and "germ-proof" structure. Tissue, epidermis, 

 replaced later by cork, covering stems and leaves, but merg- 

 ing on young roots to the root hair layer. This physi- 



