GRASSES OF IOWA. 



Fig. 3. Roots of Foxtail (plerome). 



(Setaria viridis ) (King.) , , ., i \ 



cortex (periblem). 



furnished to the seed the root pushes through the covering of 

 the seed and enters the soil. At this early stage it is easy 

 to make out a conical tip known as the root-cap. This 

 consists of loosely arranged parenchyma cells. These dells 

 are not alike, but differ in size and shape. It is easy to observe 

 that the cells at a point back of the tip converge; they are des- 

 tined for a different purpose from the cells above and below. 



This point is known as the growing point 

 or punctum vegetationis. All other cells 

 are derived from the growing point. 

 Below, the cells of the root-cap are 

 formed. These are continually being 

 removed as the roots push through the 

 soil. The root-cap is purely protective 

 in its function. In the center occurs 

 what is known as the central cylinder 

 On either side occurs the 

 The outer layer is 

 the epidermis (dermatogen). These cell walls are more or less 

 mucilaginous. All of these young cells contain an abundance 

 of protoplasm and a distinct nucleus. At a little distance back 

 of the tip, the epidermal cells form root hairs. These are at 

 first straight and have delicate walls, but as they increase in 

 length they become tortuous and insinuate themselves among 

 soil particles, the grains of sand adhering owing to the muci- 

 laginous character of the cell walls. Owing to their intimate 

 contact with the same, it is difficult to remove the soil. The 

 older root hairs are continually being re- 

 placed as the root increases in length. A 

 cross section through an older root shows 

 that it is made up of three parts: the outer 

 epidermis; the central axis or cylinder, con- 

 taining the fibro- vascular elements; and be- 

 tween the two, the parenchymatous tissue. 



The central cylinder is bounded by the 

 endodermis and a peripheral layer. The 

 cells of the endodermis are without inter- 

 cellular spaces, and are more or less plicate. 

 In secondary roots the rear walls are thick- ng.i. section of 

 ened somewhat like a crescent. The bun- do*sp|rm^®pj/pium- 

 dles are arranged in a radial manner, the cluiicie?pJ-!primary 

 woody elements (xylem) occupy the center J°°^shelth}"(K'ing'> 



