30 GROWTH AND WORK OF PLANTS 



seed production or for starting new plants. In the sweet potato 

 the food is stored as sugar, while in the dahlia it is stored as 

 inulin, common in many composites. In the natural condition 

 it is in solution, but by prolonged immersion of the tissue in 

 alcohol the inulin is precipitated in the form of sphaero crystals. 



52. External structure near and including the root tip. 

 The principal general features in the structure of roots can be 

 obtained in the study of the roots of seedlings. The form of the 

 extreme tip is conic, and it consists of a group of cells which are 

 loosely held together, especially on the outer surface, where 

 they constantly become free and die. This is the root cap, and 

 it protects the delicate growing cells just back of the tip. A 

 splendid example of the root cap is afforded by the screw pine. 

 It is very large and can very readily be seen on the prop roots 

 before they reach the soil. Another interesting example is found 

 on the roots of the water hyacinth common in some of the streams 

 of Florida and sometimes grown in aquaria in greenhouses. The 

 root cap is long and can be very easily pulled from the root and 



then slipped on again. Just back 

 of these loose cells and still near 

 the tip, the surface of the root is 

 smooth and the cells are united 

 closely and firmly. The outer layer 

 of cells, somewhat elongated and 

 rectangular, forms the "skin" or 

 epidermis as it is more properly 

 called. When the young root is 

 transparent this layer can be traced 

 some distance underneath the root 

 cap where it arises from the meri- 

 Fig. 35- stem (see paragraph 54), and the 



Germinating wheat, showing root hairs. ^.^ Qf ^ ^^ ^^ ^ 



also be seen (fig. 37). A little farther back from the tip of the 

 root some of the epidermal cells grow out into the long root 

 hairs. The root hairs occupy a rather definite area of the root 

 a little distance back from the tip. As the root elongates new 



