CELLULAR TISSUE. 21 



has received the name of Lignine (lignum, wood), or solerogen 

 (sxTijjgos hard, and ysnanv to generate). 



12. In the lax parenchyma or tissue of plants, where the 

 cells retain in part their spherical or cylindrical figure, in 

 proportion as they retain that figure when they mutually 

 impress each other do they necessarily touch each other at 

 certain points only, leaving vacant spaces formed between 

 the cells; which, when they occur as separate cavities, are 

 called intercellular spaces ; but when they follow the course 

 of the tissue, and occur as continuous and tubular sinuosi- 

 ties, they are termed intercellular passages. 



13. Sometimes these intercellular spaces become the re- 

 ceptacles of the peculiar secretions of the plant. The trans- 

 parent dots seen in the leaves of the orange, the myrtle, and 

 the St. John's-wort, when they are held up to the light, are 

 produced by the presence of oil in the intercellular spaces 

 of the parenchyma, whilst the transudation of the milky 

 secretions into the intercellular passages forms the so-called 

 vessels of the latex. 



14. These consist of long, irregularly-branching, and anas- 

 tomosing tubes, of extreme tenuity and transparency, having 

 a diameter of about l-1400th of an inch, and forming, by 

 their union, an anastomosis or network, like the veins of 

 animals. They ar.e visible only under a powerful microscope, 

 and were formerly thought to be cells united in a linear 

 series, their septa being obliterated; but, at a very early 

 period, the milky secretions of the plant may be seen circu- 

 lating through the intercellular passages, and from these 

 secretions the membrane forming the continuous and anasto- 



