YOSEMITE NATURE NOTES 



the acorns are conspicuous features, 

 and when its foliage assumes the 

 rich golds and browns characteristic 

 of the season. Likewise the Pacific 

 dogwood is distinctive, not only in 

 the spring when it is festooned with 

 numerous large white blossoms, but 

 also in the fall when it bears clusters 

 of bright red seeds, and when the 

 foliage takes on a characteristic au- 

 tumnal red. Other species possess 

 outstanding characters of a similar 

 nature which attract their share of 

 visitor interest. The foliage of the 

 California laurel gives off a penetrat- 

 ing, pungent odor when bruised; the 

 holly-like leaves of the canyon live 

 oak rarely fail to attract attention; 

 and the quivering of the long-stem- 

 med leaves of the aspen is a familiar 

 sight in the "high country." 



Leaf Fall and Autumnal Color of 

 Foliage 



With few exceptions the broad- 

 leaved trees of Yosemite National 

 Park lose their leaves in the fall. This, 

 and its related phenomena, is the 

 result of the tree's preparation for 

 winter for as the season approaches, 

 deciduous trees must necessarily be 

 ready to withstand its rigors. These 

 preparations are largely to prevent 

 excessive transpiration, since abnor- 

 mal loss of water may result in the 

 death of the tree. They are respon- 

 sible for the vivid fall colors charac- 

 teristic of the foliage of many decidu- 

 ous trees, the annual loss of foliage, 

 and the development of many fea- 

 tures useful in winter identification. 



During the summer the leaves 

 serve as places of food manufacture. 

 As autumn approaches, practically 

 everything of nutritive value to the 



tree is gradually transferred to other 

 parts and the leaves soon become 

 mere skeletons, their cells containing 

 only pigments which are of no further 

 use. These pigments are responsible 

 for the fall colors. 



Leaf fall, most obvious of the de- 

 ciduous tree's preparation for winter, 

 is anticipated weeks before the oc- 

 currence of this event. By midsum- 

 mer a layer of loose cells begins to 

 form across the base of the leaf stem. 

 When complete it extends entirely 

 across the stem at the point where it 

 joins the twig, except for the vascular 

 bundles which must necessarily re- 

 main open to facilitate the transpor- 

 tation of food and moisture. Subse- 

 quently an additional layer of corky 

 cells form under the one previously 

 developed. With the advent of fall, 

 with its rains and frosty nights, small 

 crystals of ice develop between the 

 two cell layers. These exert a prying 

 action which snaps the leaf from the 

 twig. The leaf scar, which would 

 otherwise have remained as an open 

 wound, is protected by the corky cell 

 layer. 



However, this is but one episode 

 in the tree's preparation for winter. 

 By midsummer the buds, conspicuous 

 on the naked twigs in winter, ore 

 formed. These contain the rudimen- 

 tary foliage or flower parts destined 

 for development in the following 

 summer which are protected by 

 scales, waxes, gums, or hairs. In ad- 

 dition growth is retarded and finally 

 ceases, the recently formed tissues 

 are "hardened," and the bark of the 

 twigs and branches is increased in 

 thickness through the addition of 

 corky tissue. 



