CONTINUED EMBRYOLOGY 



13 



each 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 possibility 

 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 

 until the distal apex is arrested or 

 destroyed. Some distance down the 

 stem a zone will be found marked transversely by many narrow scars 

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

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

 zones may be found successively lower down, marking the limits 

 of the increment of growth of earlier years. Each year's growth 

 leaves its record on the outer surface of the branch. Thus, passing 



Fig. 6. 



Twig of Horse Chestnut in winter, 

 indicating the end of the increment of 

 growth of 1916, the limits of increment 

 of 1917, and the bud to be expanded in 

 1918, with scars of bud-scales and of 

 foliage leaves, and axillary buds and 

 lenticels. Natural size. 



