232 - Multicellular Plants 



exhausted, the young plant becomes entirely 

 sell-supporting. 



Further Stages of Development. New 



branches and leaves originate from buds, 

 which are generally situated at the apex and 

 on the sides of the stem and its branches. 

 The terminal buds provide for an elonga- 

 tion of the stem or branch; and the lateral 

 buds give rise to more branches (Fig. 12-36). 

 The lateral buds are also called axillary buds 

 because each one lies in a leal axil — that is, 

 in the acute angle between the stem and the 

 stalk, or petiole, of a leal (Fig. 12-36). The 

 immature bud is merely a conical mass of 

 meristem tissue; but in mature buds (Fig. 

 12-37), partially developed leaves are present. 

 The young leaves first appear as lateral out- 

 growths from the young bud; but gradually 

 the young leaves overgrow and envelop the 

 growing point of the bud (Fig. 12-37). Some 

 buds remain in a dormant stage, without 

 further development, for some time — as is 

 true of the winter buds of trees. Such rest- 



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LEAF SCAR 



AXILLARY BUD 



LENTICEL 



SCALE SCARS 



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Fig. 12-36. Horse-chestnut twig in winter condition. 



Fig. 12-37. Bud of Elodea, a naked bud. The growing 

 point (A) lies at the top; and along the sides secondary 

 buds (B and D) alternate with the leaves (C). 



ing buds are usually of the protected type; 

 that is, they possess an outer covering of 

 horny modified leaves, called scales, which 

 envelop the delicate embryonic leaves and 

 the growing point of the bud (Fig. 12-36). 



The growth of a leaf, unlike that of most 

 stems and roots, is limited; that is, the leaf 

 stops growing when it reaches a certain size 

 and form. The short but rapid growth of a 

 leaf subsides first in the basal portion, and 

 only later does the apex of the leaf reach its 

 maximum size. 



In perennial plants, the primary stem may 

 continue to elongate year alter year as the 

 terminal bud goes through its alternate peri- 

 ods of rest and growth. Branches of the stem 

 originate from the axillary buds, but many 

 axillary buds do not develop unless the 

 terminal bud is cut off or otherwise de- 

 stroyed. Accordingly, the general shape of 

 many plants can be modified quite drasti- 

 cally by pruning. 



The growth of roots differs somewhat from 

 that of stems. Roots never give rise to buds 

 or leaves, and the growing point is generallv 

 covered by a rootcap. Secondary branches of 

 the root originate at various levels above the 



