CH. IV, 10] FOLIAGE-BEARING STEMS 



183 



so much of the time in the leeward position that it acquires 

 a permanent set that way (Fig. 125), though the result is 

 complicated by the greater transpiration, and consequent 

 less growth, on the windward side. The leverage of the 

 winds is felt most at the base of the trunk, which explains 

 the need for the buttresses above mentioned. There is 

 evidence to show that these buttresses, like the brackets 

 and excentric growth of the branches, develop in irritable 

 self-adjustment to the stim- 

 ulus of the strains there felt, 

 in precisely the same way 

 that leaves and stems turn 

 phototropically to light, or 

 stems hold themselves up- 

 right in adjustment to grav- 

 itation. 



Between stems and 

 branches no structural dif- 

 ferences exist, the word 

 " branch," as we use it, 

 being merely an abbrevia- 

 tion for " branches of the 

 stem." For the most part 

 all of the branches of a given plant are structurally alike, but 

 sometimes they are not. Thus in fruit trees, some branches 

 make extremely little growth in length each year, while their 

 buds form flowers and fruits with the least possible stem; 

 and such branches are the familiar FRUIT-SPURS. Again, 

 some of the branches on a plant may be limited in growth 

 and assume flat forms, as in cladophylla elsewhere described 

 (page 195), the remaining branches having the ordinary 

 form. An even more familiar case of special branches is 

 found in flowers, which are morphologically modified branches 

 including sexual parts. In a few cases, trees form a certain 

 absciss-layer across the bases of some of their young branches, 

 producing the result of a natural pruning. 



FIG. 125. A yellow Birch, ex- 

 posed to winds from one direction 

 during the growth season. (Drawn 

 from a photograph.) 



