Vegetative Organs of Vascular Plants 123 



pot-boiind plants and the plants can be repotted without apparent 

 damage. Mitotic activity is usually rapid during the early forenoon. 

 Maize cytologists favor a formula that is practically identical with 

 Craf III. Choose a stain by the criteria discussed in connection with 

 the onion. 



J'icia I aba, the horse bean, has 12 large chromosomes, 2 of them 

 about twice as large as the others. Obtain root tips by sprouting 

 seeds in a moist chamber or from plants grown in pots of sphagnum. 

 Kill in Nawaschin or in Craf II, and stain as with onion. The radicles 

 of many other legumes also are easy to obtain and to process. Sections 

 may be stained for either histological or cytological details (Fig. 

 13.2) , or a good compromise may be obtained with safranin-fast 

 green. 



The common trailing Zebrina grown in greenhouses has large 

 chromosomes. Obtain root tips from cuttings rooted in sand. The 

 periodicity is an uncertain factor, and the worker must chance ob- 

 taining abundant mitotic figures. Bouin's solution and Craf II usually 

 give acceptable results. 



APICAL MERISTEMS OF THE STEM 



The origin of the tissues of the stem and of lateral organs on the 

 stem is revealed by a study of the meristematic tip or apex of a stem. 

 This growing point may be foiuid at the tip of a growing axis, or in 

 a dormant terminal or axillary bud. One of the easiest subjects to 

 handle is the shoot from the sprouting kernel of corn. Sorghum, oats, 

 and other small grains also may be used, but the coleoptile is small 

 and not so easy to handle as that of corn. The growing point of these 

 Gramineae is a broad dome, from which the leaf primordia arise 

 as lateral protuberances. Successive leaf primordia are laid down 

 dining this period of rapid growth and may be seen in graded order 

 in a good median longitudinal section. Transverse sections show the 

 lateral extension of the meristematic margins of the leaf primordia. 

 The oat sprout develops axillary buds earlier than do the other sug- 

 gested grasses. 



To obtain growing points of stems of Zea, germinate the corn in 

 sphagnum or sand. When the coleoptile is approximately 5 cm. long, 

 cut out a section 5 mm. long at the coleoptile node (Fig. 13.1/1). 

 This region, which contains the growing point, can be located easily 

 by holding the shoot before a bright light — the region of the coleop- 

 tile node and compact growing point area is dark. Older seedlings 

 show more advanced axillar) buds than the young sprouts. One 



