126 General Botany 



On many varieties of apple and pear trees, flower buds are 

 usually borne on the ends of " spurs " or short, slow-growing 

 twigs. When the spur ends in a flower, the further growth of the 

 shoot depends upon the development of a lateral bud. Com- 

 monly the spurs produce flowers in alternate years. When once 

 a branch produces flowers, it continues to do so and its growth 

 rate is usually much slower than that of the vegetative branches. 

 Because of the successive development of lateral buds, spur 

 branches are crooked and the intervals between terminal bud 

 scars are short. 



Lenticels. AH living cells require energy. This is mostly 

 obtained from respiration. Therefore, in addition to a constant 

 food supply, the cells of the stem must have access to oxygen. 

 As in the leaves the oxygen is supplied through the intercellular 

 spaces, so in stems there must be sufficient intercellular spaces 

 to permit oxygen to diffuse inward and carbon dioxide to diffuse 

 outward. There must also be openings through the epidermis 

 or bark to connect these intercellular spaces with the outside 

 atmosphere. 



The young green stems of all plants have stomata. Perennial 

 stems, however, soon develop a corky layer beneath the epidermis, 

 which cuts the cells in the interior of the stem off from the 

 stomata. While this layer is developing, masses of round, loose 

 cells form beneath some of the stomata, pushing out and tearing 

 the epidermis above them. These open places are the lenticels. 

 They permit gas exchanges, and in older stems take the place of 

 the stomata. The lenticels of most twigs of trees and shrubs are 

 closed in the late autumn by the growth of a thin layer of cork 

 beneath them. The following spring loose cefls are again formed 

 at the same point, the cork is burst open, and the lenticels again 

 permit gas exchanges. Apparently water influences the develop- 

 ment of open lenticels. If a willow twig is placed in water, the 

 submerged lenticels enlarge greatly. Perhaps a similar condition 

 effects the opening of lenticels in the spring. 



