STATING THE CASE. 



23 



with green grains called Chlorophyll, and these are 

 chemical laboratories in which the most important 

 work in the whole organic world is carried on. We 

 shall see more about them presently. 



In the upward process of the growth of plants 

 many of the cells 

 change their shapes, 

 and also transform 

 their cell contents. 

 The cells get drawn 

 out into long, taper- 

 ing vessels which are 

 packed against each 

 other as if they were 

 spliced. In this way 

 the tissue becomes 

 very strong. It is 

 converted into true 

 wood by the internal formation of a substance called 

 lignine. In herbaceous plants, however, the latter is 

 seldom formed except in the veins, etc., of leaves, 

 hence the reason why the latter can be " skeleton- 

 ised." The cells of the roots of plants seldom 

 secrete chlorophyll ; their sap, instead, is usually of 

 an acid characterj especially in the fine rootlets. 



The cells of plants are as particular about the 

 material used in their construction as a brickmaker 

 who is anxious to supply really good bricks is to 

 the kind of clay he uses. Only certain materials 



Fig. 6. — Cells of a Potato magnified, showing 

 Starch-Brains in their Interiors. 



