378 Agricultural Experiment Station, Ithaca, N. Y. 



a plant has been entirely girdled, as frequently occurs with or- 

 chard trees, and as is sometimes purposely done with grape- 

 vines when particularly large fruit is desired, it is interesting to 

 note that the plant makes little attempt to cover the wound from 

 below, but the healing takes place from above. At the same 

 time the foliage does not wilt as if suffering from water, unless 

 the cut has been made very deep, but it frequently remains green 

 and apparently healthy for a long time. 



The above facts lead to but one conclusion: the sap upon en- 

 tering the plant rises through the inner tissues to the extremities 

 of the branches, or to the leaves; from here it descends, choosing 

 for its path, however, the tissues which lie between the outer bark 

 and the wood. The part through which the sap rises is well 

 known under the name sap-wood. In many plants this wood is 

 very conspicuous in sections of the stem on account of its light 

 color. The sap descends through what for convenience may be 

 loosely termed the inner bark, which consists of the soft tissues 

 that lie directly underneath the hard, corky covering of the stem. 



Endogenous plants, such as the palm, corn, and others, do 

 not have these tissues separated from each other as above de- 

 scribed. The tissues which correspond to the sap-wood and to 

 the inner bark are arranged in the form of long, slender, thread- 

 like bundles which are readily distinguished as coarse fibers, 

 thinly scattered in the pith as seen in corn stalks, and more 

 thickly at the edges of the stems. The sap rises and descends in 

 each of these many bundles of fibre, so that the girdling of this 

 class of plants is not followed by such abnormal growths as 

 occur on our fruit trees; the primary result of such injury is that 

 the amount of sap which reaches the foliage is reduced in pro- 

 portion to the number of these fibres which are cut. 



The reason why the sap passes directly to the foliage before it 

 returns to the growing parts of the plant is obvious; in the tissues 

 of the leaves the crude sap is acted upon by various agents, with 

 the result that the nourishment which was carried to the leaves is 

 made available for use by growing cells. The process of chang- 

 ing the unavailable food to that which is of use to the plant is 

 known as assimilation; the green portions of the plant are the 

 only parts in which this change can take place, and it can pro- 



