and Laboratory Methods 2607 



phloem bundles alternating with the xylem, the beginning of the stelar cambium 

 passing between the xylem and phloem, the endodermis or bundle sheath, and the 

 broad cortex with a superficial layer of cells known as the piliferous layer. 



2. Cut cross sections of a somewhat older root which has turned brown, 

 mount, and sketch the entire section under low power. Represent the following 

 structures : the central strand of xylem composed of wood vessels and smaller 

 cells, the stelar cambium, the band of phloem consisting of several kinds of 

 cells, the remains of the endodermis, and the cortex and piliferous layer turned 

 brown. 



(b) Embryonic Root Tip. 



1. Carefully remove the hard parts around the base of the embryo in a 

 grain of corn (Zea mays L.) and with a razor cut longitudinal sections of the radi- 

 cle of the dormant embryo. The corn may be soaked in water for a while before 

 cutting the sections, though this is not necessary. 



2. Mount the central sections in water and sketch under low power. Note 

 the following embryonic tissues : the outer scutellum, the root-sheath of a dark 

 appearance inside of the scutellum, and the root tip inside of the root-sheath. 

 The root tip is made up of the root-cap (of a light color), the dermatogen (a 

 layer of large cells inside of the cap), the dark layer of periblem, the central light 

 plerome, and the growing point at the tip of the plerome. Outside of the der- 

 matogen, at the apex of the root is the formative tissue of the root-cap, known 

 as the calyptrogen. It will probably not be distinct enough in these sections to 

 trace out, but its position should be noted. 



(c) Root Hairs. 



1. Sprout grains of corn on moist blotting paper in a box or under a bell- 

 jar ; after a few days the roots will be covered with root hairs. Sketch under 

 dissecting microscope. 



2. With a scalpel cut off some of the epidermis containing root hairs, mount 

 and examine under high power. Draw and describe. 



3. Under low power examine roots of young seedlings planted in soil and 

 note the relation of the root hairs to the soil particles. 



XCV. Lenticels. 



1. Examine and sketch the bark of a green and of a year-old elder stem 

 {Sambucus canadensis L.) showing the surface covered with lenticels. 



2, Cut cross sections of the bark, mount, and examine under low power. 

 Sketch one of the lenticels. How and where do they originate? 



XCVI, Starch, Cellulose and Lignin. 



1. Cut a potato and scrape off some of the cells. Mount in water and 

 study under high power. Draw some of the large starch grains present, show- 

 ing the hilum and the stratified structure. 



2. Place a drop of iodin solution beside the cover-glass and watch its effect 

 on the starch. What is the color reaction ? 



