How Plants Resist Hostile Forces Around Them 261 



the young parts, and then curve vertically upwards again to 

 bear the new leaves, — the whole stem melting away, as it were, 

 to a spray of such branches. This is the form prevailing in most 

 of our deciduous trees, as the reader can see for himself by 

 examining the tracery of 

 Oaks, Maples, Elms, or 

 Chestnuts when projected 

 against the winter sky. 

 Such a sigmoid form of 

 the branches affords them 

 the best possible anchor- 

 age in the trunk with the ^r^ on t^ • r u . u r .u 



_ _ riG. 90. — Tracings from photographs of the same 



minimum of leverage on Balsam Fir, in the natural position and inverted, 

 . . , . illustrating a point explained in the text. 



their heaviest parts, while 



providing enough spread and a vertical tip for support of the 

 foliage. If, now, we apply this sigmoid mechanical modification 

 to the theoretical form of our photosynthetic tree represented in 

 figure 7, we obtain the form illustrated herewith (figure 91). 

 This theoretical form, modified by some minor and largely acci- 

 dental circumstances, is very nearly realized in the noble Oak 

 shown in figure 8, and by many of our common deciduous trees. 

 The chief difference consists only in this, that whereas the 

 theoretical tree is hemispherical, the actual kinds are often 

 ovoid, cylindrical, or top-shaped, — in obvious adaptation, as I 

 think, to a diminution of the excessive gravitational leverage 

 that accompanies too extensive a spread. 



We pass now to a second of the greater environmental in- 

 fluences hostile to plants, namely excessive light. The reader 

 does not need to be told that light, and in large quantity, is in- 

 dispensable to plants for their photosynthetic work; but it is an 

 important physical fact that the amount they can thus use has a 

 limit, above which any increase is not only useless but positively 

 harmful. And that limit is often surpassed in the open sunlight 

 of summer. However, not all of the mani-colored rays that make 



