CONTINUED EMBRYOLOGY 



13 



which the oldest are nearest to the base and the youngest distal, 



while those at the extreme tip are closely grouped so as to form a 



terminal bud. Further, in the axil, or angle between the base of 



€ach leaf and the stem, a fresh bud may appear, which repeats the 



chief characters of the terminal bud. 



Each bud is capable of developing into 



•a lateral branch similar to the main 



shoot, and so on. The increase in 



number of shoots or of roots is in fact 



on a very prolific scale. In herbs, such 



as the Sunflower, Bean and Castor Oil, 



this mode of development is not carried 



far ; but still the unlimited possibiHty 



exists in the plan of their construction. 



It is precisely the same scheme carried 

 out further which gives rise to shrubs 

 and trees. In some of them the develop- 

 ment upon this plan may be continued 

 for centuries, and the organism may 

 attain very great size and a high com- 

 plexity of branching. The result of such 

 continued growth may be very well 

 studied on the twigs and branches of 

 trees in winter, when the leaves have 

 fallen, or in the spring when the winter 

 buds are bursting. For instance, on the 

 Horse Chestnut (Fig. 6), each shoot is 

 terminated by a bud, composed of exter- 

 nal bud-scales, which enclose the closely 

 folded foliage leaves awaiting expansion 

 in the succeeding season. The woody 

 stem below is marked by opposite pairs 

 of semicircular scars, where the leaves of 

 the preceding season fell away in autumn. 

 Immediately above each scar an axillary 



bud may be seen, which is capable of developing into a new branch ; 

 but frequently these remain dormant as a reserve of branches to 

 be developed only if required. Some distance down the stem a 

 zone will be found marked transversely by many narrow scars close 

 together. This marks the lower limit of the preceding year's growth, 

 and the scars are those left when the bud-scales fell away 



Fig. 6. 

 Twig of Horse Chestnut in winter, 

 indicating the end of the increment of 

 growth of 1916, the limits of increment 

 <3f 1917, and the bud to he expanded in 

 igiS, with scars of bud-scales and of 

 foliage leaves, and axillary buds and 

 lenticels. Natural size. 



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