298 ASSIMILATION. 



793. These autumnal changes have been compared, not in- 

 aptly-, to those belonging to the ripening process in colored fruits ; 

 but this general statement of similarity must not disguise the 

 fact that in the ripening of fruits special chromoplastids play the 

 chief part, whereas in the leaf before its fall there is a breaking 

 up of the protoplasmic basis of the granules of chlorophyll,^ pre- 

 paratory to the withdrawal from the leaves into the plant of the 

 useful products of disintegration. 



The changes during disintegration may involve (1) both color 

 and form of the gi-anules at one and the same time, or (2) the 

 change in color may precede that in form, or (3) the latter mav 

 occur first. 



794. In general, the reddish coloring-matters are found in the 

 cell-sap of the colored leaves, the yellow in the substance of the 

 disintegrating grain, and, finally-, the brown in the modified 

 character of the cell-wall itself. 



795. That frost is not essential to the production of the leaf- 

 colors of autumn is plain from the widely known fact that many 

 leaves undergo precisely these changes of color long before any 

 frosts appear. It is generally believed, however, that freezing 

 ma^- somewhat hasten the process of chlorophjU disintegration 

 •which underlies all the changes. 



The fact is generally recognized that the autumnal colors, 

 crimson and scarlet, are more brilliant in the cooler portions of 

 Amei'ica than those which characterize the foliage in Europe, 

 and it has even been remarked that the leaves of American 

 ti-ees cultivated in Europe do not undergo such marked changes 

 of color as individuals of the same species do in their native 

 habitat. This has been accounted for on the ground that there 

 is less humidity in the atmosphere of eastern America ; but this 

 explanation is not satisfactory, and exact observations regarding 

 the relative brilliancy of color are wholly wanting. 



796. Chlorophyll in evergreen leaves. At the approach of 

 cold weather the leaves of evergreens undergo, according to 

 Mohl,^ certain changes of color. Kraus ' recognizes two types 

 of change: (1) the leaves become greenish brown, as in most 

 Conifers, or (2) thej- take on a red color on the upper side, as 

 in Mahonia and some species of Sedum. According to him, in 

 leaves of the first type the chlorophyll granules become disinte- 



' Sachs : Die Entleeiung der Blatter im Herbst, Flora, 1863, p. 200. 



^ Vermisohte Schriften, 1845. 



^ Sitzung.sb. der phys.-med. Societat zu Erlangen, 1871, 1872, 



