46 STRUCTURAL BOTANY 



The different kinds of cells are not distributed 

 uniformly all through the substance of a plant. They 

 have a definite arrangement, and the various kinds of 

 cells are associated together so as to form more or 

 less sharply defined layers or strands. Such an 

 association of similar cells is termed a tissue. Certain 

 kinds of tissue, again, are generally associated 

 together, and such a combination of tissues is called 

 a tissue -system. 



We are now in a position to undertake the study 

 of the internal structure of the Wallflower. We will 

 begin with the stem, and will make ourselves acquainted 

 with the tissues of which it is composed. 



a. Structure of the Stem 



The stem of the Wallflower is traversed lengthways 

 by a number of strands of rather tough, stringy tissue. 

 It is possible to dissect away the soft substance in 

 which they are embedded, and so lay bare the strands 

 themselves. If a stem be allowed to rot, it is this 

 stringy part which will longest resist decay. Most 

 people will have noticed at' some time or other an old 

 rotten cabbage stalk, in which all the soft parts have 

 perished and only a network of hard, woody, inter- 

 lacing strings is left behind. These strands are the 

 vascular bundles. They constitute one of the systems 

 of tissue of which the higher plants are composed. 

 Their great function is to serve as conducting channels 

 through which the various food-materials pass. In 

 most cases they are also of importance as forming a 

 supporting skeleton for the plant. 



