THE STRUCTURE OF PLANTS. 



ii 



though only acting upon the most minute particles (mole- 

 cules), are still sufficient to produce the ascent of the sap 

 from the lowest root fibre to the highest twig of the most 

 gigantic tree. 



iiillllWHi: 



bed e 



The above diagrams illustrate various ducts or vessels 

 — b, annular fibres ; e, spiral fibres ; d, scalariform vessels ; 

 e, lactiferous vessels containing a fluid called latex, abun- 

 dant in the dandelion, thistle, and other composites. 



By the increase of cells, in number and size, is produced 

 the growth of the plants. The cells, when mature, mys- 

 teriously exhibit first a slight contraction in the middle, 

 growing gradually deeper, until at last completely divided 

 into two, in each of which the same process is repeated 

 successively for a certain time, viz. as long as the cell- 

 wall remains thin enough. As the older ones thicken by 

 deposition of lignine, each dies, and only serves afterwards 

 to strengthen and support the organism, while new and 



