249 



[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS 



stem tip under the microscope is seen to have all the structures found in 

 scaly buds except the scales ( Fig. 32 ) . In addition to the stem tip there 

 are young leaves, with additional buds in their axils. These small axil- 

 lary buds within the larger bud may consist either of stem tips bearing 



AXILLARY FLOWER PRIMOROIUM 



•AXILLARV BRANCH PR1^K3RDIUW 

 BASE OF LEAF 



eJ 



-^^ 



r A 1 



B 



Fig. 83. Diagrams of longitudinal sections of terminal buds of coleus plants. 

 In A the axillary buds, except the lowest pair, consist of stem tips bearing flower 

 primordia of various ages. In B the axillary buds consist of stem tips or of stem 

 tips bearing leaf primordia. 



leaf primordia or of stem tips bearing flower primordia, depending on 

 whether the plant is continuing vegetative growth or is changing to 

 flower formation ( Fig. 83 ) . This change to a reproductive state in coleus 

 is accompanied by a change in the shape of the temiinal bud and the 

 upper pair of leaves. 



The youngest bud primordia consist only of stem tips. Buds of many 

 embryos and those in the "eyes" of potato tubers often consist only of 

 stem tips. Apical growth is particularly characteristic of stems and roots. 



A bud then may be defined as a stem tip having extremely short 

 internodes and bearing young leaves or flowers, or both. It may or may 

 not be enclosed by bud scales. Stem segments bear leaves but once, 



