THE SECRETARY'S* PORTFOLIO- 199" 



AUTUMN LEAVES. 



What is the cause of these beautiful and endlessly diversified colorings of 

 the leaves in autumn? The immediate cause is the lessened activity of their 

 pores, or breathing organs, owing to the feebler illuminating power of the sun, 

 which is nearer the horizon at midday, shines upon it more obliquely, and sets 

 sooner. It has been observed that plants bend from the red or calorific rays of 

 the sun's spectrum, instead of toward them, as they are found to do in regard 

 to every other ray. From this curious circumstance the inference has been 

 drawn that the effect of these rays is to check the vegetative processes, and 

 favor the formation and growth of the blossoms and fruit in which the activity 

 of every plant terminates. Light is only a secondary cause of activity in blos- 

 soms, while it is the principal cause of activity in leaves. If the deficiency of 

 light is compensated by an increase of heat, blossoms may be made active even 

 in total darkness, while the leaves will present only a very imperfect vitality. 

 Thus we see that heat favors the production of the blossom, while it has little 

 or no effect upon the leaves. And in accordance with this, it has been further 

 observed that the heat rays are more powerful in autumn than at any other 

 season ; while the actinic rays which stimulate germination, and the luminous 

 rays which are recpured for the formation of the tissues and their products, are 

 proportionately diminished. The rise of the sap in spring is quickened by the 

 actinism which is then most abundant in the solar bsams ; the secretion of 

 carbon in summer is due to the greater predominance of light; and the rever- 

 sion of the sap, the ripening of the fruit, and the discoloration of the leaf in 

 autumn, may be traced to the increased power of the parathermic influence, 

 from which the plant is found to bend, and which appears to be a modified 

 form of heat. The distribution of plants over the surface of the earth, as 

 well as their periodic changes, are dependent upon these causes, — the greater or 

 less amount of actinism, light, or heat in the climates, and the seasons in which 

 they grow. Given the proportion of these different principles of the sunshine 

 of different countries aud seasons, and it will be easy to predict the character 

 of their vegetation and the nature of their vegetable products. 



Bearing these curious influences in mind, it is easy to understand why, with 

 the diminished light of the shorter days of autumn, the natural stimulant of 

 vegetation should be correspondingly withdrawn, and the circulation of the sap 

 through the lamina, or blade of the leaf, cease. Carbon cannot be formed 

 from the atmosphere, except under the influence of light ; and when this stim- 

 ulant is weakened, the vital powers of the leaves are proportionately dimin- 

 ished, and the affinity which they have for oxygen acts with little opposition. 

 The great natural life-process of deoxidation not only stops, but is reversed. 

 Oxygen is absorbed, and carbon, the product of combustion, is given off ; and 

 the chlorophyl, or that carbonaceous product, which gives the green, healthy 

 color to leaves, is slowly oxidized, and rusts away into the various tints of 

 autumn. The hues of the foliage become brighter as the process of oxidation 

 goes on ; those of early autumn being dull and dark greens, passing later in 

 the season into the bright yellows and reds, which make our woods full of sun- 

 shine, and their depths more brilliant than the open fields, or the mountain- 

 tops. The coloring of leaves and fruits undoubtedly proceeds from the same 

 cause, viz. : a cessation of the deoxidating process in the contents of the super- 

 ficial cells. The leaves ripen in the same way that the fruit does ; and the 

 chemical change which the chlorophyl in both undergoes, indicates that 



