of the gall depends upon their number. In some parts of the root 

 will be found minute pimple-like excrescences, usually of a yellowish 

 color, just large enough to contain a single worm. From these the 

 galls and number of worms contained varies indefinitely. 



In order to get an idea of these abnormal root growths, let us 

 first briefly consider the normal structure of the root in a plant like 

 the cucumber. If such a root be cut across with a sharp knife there 

 can readily be distinguished on the cut ends two different tissues or 

 parts. The central part of the section is occupied by a more or less 

 star or cross shaped portion differing in color and appearance from 

 the other tissue which surrounds it. This is called the central cylm- 

 der, and the other part the cortex. Both are composed of variously 

 formed celts, as are all parts of the plant. The cortex, (PI. VI., fig. 

 2, c.) consists of comparatively large, thin walled cells which make it 

 a sort of spongy tissue, the principal function of which is to absorb 

 water from the soil. In this water are dissolved the substances fornr 

 ing the food of the plant. The central cylinder, (PI. VI., fig. 2, p.) 

 consists of several tissues, each having its particular structure and 

 function. Its elements are mostly composed of cells of a firmer, 

 thicker structure than those of the cortex and thus it serves to give 

 the root its strength and stability, just as the woody portions do in 

 the stem. Among the elements of this central cylinder one of the 

 most important is a tissue composed of large, long, thick-walled, 

 tube like cells, connecting end to end longitudinally to form passages 

 from the root up through the entire plant to the leaves, (pi. VI., fig. 2, 

 d.). Through these vessels, which are called di/cts, the crude sap, 

 i. e. water containing nutritive substances in solution, coming in 

 through the cortex from the earth is carried up to the stem and 

 thence to the leaves where it is transformed under the influence of 

 sunlight into plant substance. We may, in a very general way, con- 

 sidering only the function of taking water from the soil, liken such a 

 root to a bundle of tubes composed of some material through which 

 water can pass, enclosed in a covering of spongy material ; the tubes 

 of course representing the ducts of the central cylinder and the spongy 

 material the cortex. Imagining such a contrivance to be placed in 

 water, it can readily be seen how the water might soak through the 

 outer layer into the tubes and thence be carried wherever an impelling 

 force might direct it. Such a force is supplied in the plant by the 

 so-called root pressure, the force which circulates the sap. 



