DEVELOPAIExNT OF LATERAL ORGANS 563 



midrib primordium. Subsequent divisions, enlargement and maturation of 

 these cells result in the development of all of the mesophyll tissues, including 

 the lateral veins. The epidermis increases in area as a result of continued 

 division and enlargement of the epidermal cells. Cell divisions cease first in 

 the epidermis, followed in order by the middle and lower mesophyll, and the 

 palisade layers. The tissues of the lateral veins may continue to develop long 

 after cell division has stopped in other parts of the leaf. 



Although cessation of cell division occurs first in the epidermis, enlarge- 

 ment of the cells in this layer continues longer than in any other tissue of the 

 leaf. This results in pulling the cells of the lower layers of the mesophyll 

 apart and in the development of their typical spongy condition. For a similar 

 reason there may also be a limited development of intercellular spaces in the 

 palisade layers. Litercellular spaces do not develop markedly until the leaf 

 has attained one-fourth to one-third its final size. 



In the axil of each embryonic leaf a mound-like lateral meristem develops 

 (Fig. 117). This becomes a lateral bud which is essentially a rudimentary 

 side branch. In many woody plants terminal buds also form annually at the 

 tip of each stem axis. Growth from these buds is usually resumed with the 

 advent of the next growing season. In temperate zone woody plants these 

 rudimentary stem tips are encased in bud scales, which are shed only when 

 and if growth of the stem tip is resumed. The buds of most herbaceous plants 

 are devoid of bud scales. Most plants produce a great many more axillary 

 buds than ever develop into lateral branches. Whether or not one of the 

 rudimentary stem tips which is the essential part of a bud will ever resume 

 growth depends in part on environmental factors, and in part on internal 

 conditions. 



Some of the stem meristems on most species of plants become differentiated 

 into inflorescences. In some species inflorescences develop from terminal buds, 

 in others from lateral buds. Some buds produce vegetative parts only, some 

 both vegetative and flower parts, and some only flower parts. These are 

 usually designated as vegetative buds, mixed buds, and flower buds, respec- 

 tively. A flower bud is essentially a more or less abbreviated stem tip which 

 becomes differentiated into either a single flower or an inflorescence. 



The meristematic activity of flower-bearing shoots does not usually cease 

 with the production of flowers, although elongation of the shoot often termi- 

 nates with blooming. If pollination and fertilization occur (and sometimes 

 in the absence of these processes) the ovulary of the flower and often other 

 parts as well develop into a fruit, while the enclosed ovules and their contents 

 mature into seeds. The formation of these organs involves the same three 

 morphological phases of growth as the production of vegetative organs. 



