384 



Garden and Forest. 



[Number 390. 



turned to the soil ; and the fall of the leaf is an intentional 

 phenomenon, so to speak. 



More or less moisture in summer is noticeably followed 

 by a corresponding greater or less brilliancy of color in 

 autumn. During a moist summer the cuticle of a leaf 

 remains thin and its colors are vivid. In a dry summer, 

 however, this cuticle becomes thicker and harder, in order 

 to prevent an injurious loss of vi'ater from the plant ; and 

 while bright colors may form within the leaf, they appear 

 dull because seen through the more opaque skin or epider- 

 mis. The bearing of this upon the relative brilliancy of 

 American and European colors is plain. 



It is a popular belief that autumn leaves are colored by 

 frost in some vague way. This is easily disproved, for a 

 IVIaple frequently assumes its blazing colors in hottest sum- 

 mer. Moreover, it is really hard to see any perceptible 

 acceleration of color development after frost has occurred. 

 The best-accepted explanation attributes the yellows and 

 red to the action of atmospheric oxygen upon the normal 

 green coloring matter of leaves, a process somewhat com- 

 parable to the rusting of iron. Many facts favor this view. 

 We know, by direct experiment, that the green parts of 

 plants absorb oxygen at night and exhale the same kind of 

 gas during sunUght. Now, when leaves begin to turn they 

 cease to e.xhale oxygen, but continue to inhale it, in gradu- 

 ally diminishing amount, as coloration continues. Colora- 

 tion is, therefore, correlated with retention of oxygen in 

 leaves, and chemical examination of the yellow and red 

 pigments shows they are probably made in many cases by 

 oxidation of the origmal green one. Chemically speaking, 

 the "Poison Ivy, blushing its sins as scarlet," performs 

 this touching act by disintegrating its chlorophyll into 

 compounds of the erythrophyll and xanthophyll groups. 



This green coloring matter of plants, chlorophyll, is of 

 vast physiological importance, for it is absolutely essential 

 to the process of assimilation. Fungi, naturally devoid of 

 chlorophyll, always steal their food, ready made, from 

 other plants which have made their own living. A seed 

 grown in darkness ceases growth when the food stored in 

 the seed itself is exhausted. No new food can be made 

 without sunlight, and the plant is yellow, except in a few 

 cases. This yellow color is frequently seen in grass which 

 has grown under a board, and is due to a pigment called 

 etiolin, which is always antecedent to the formation of the 

 green pigment in sunlight. 



The color of a healthy leaf is made by a multitude of 

 green granules, usually roundish, distributed in most of the 

 sacs or cells of the leaf. Each granule is essentially a par- 

 ticle of that formidable protoplasm, " the physical basis of 

 hfe," and resembles a sponge which is colorless itself, yet 

 is saturated with a green liquid consisting of oils in which 

 'the pigment is dissolved. It must not be supposed that 

 chlorophyll itself can assimilate food. A solution of chlo- 

 rophyll is incapable of absorbing carbon dioxide to form 

 organic matter from that gas and water and eliminate 

 oxygen ; but this power is possessed by each protoplasmic 

 grain in which the green pigment is lodged. 



Essential as this pigment is, its physiological action is 

 involved in doubt. A very plausible theory is that chloro- 

 phyll absorbs red rays of solar light which would most 

 injuriously heat plant tissues and facilitate the combination 

 of oxygen with them ; yet, considering the extensive re- 

 searches made upon chlorophyll by keen investigators, it is 

 surprising that we still know so little about it. It has been 

 impossible to obtain the pure pigment owing to the difti- 

 xulty of removing various oils, fats, etc., always asso- 

 ciated with it ; also because it rapidly decomposes into 

 other substances, so that the chemist is not sure whether 

 he is juggling with chlorophyll or with some of its deriva- 

 tives. We shall, however, state those best-ascertained 

 properties of chlorophyll which are important in this con- 

 nection, and will then indicate how the fundamental green 

 pigment is probably altered to form conspicuous autumnal 

 colors. 



Cambridge, Mass. /«S///6- WalSOn Folsovi. 



New or Little-known Plants. 



Agave Utahensis. 



OUR illustration on page 385 of this issue will give some 

 idea of the appearance of the vegetation that clothes 

 the bluffs and cliffs which rise from the slopes of the Grand 

 Cafion of the Colorado, in, northern Arizona, at the point 

 where the river rushes along the bottom of an abyss six 

 thousand feet deep and some thirteen miles across at the 

 plateau level. In the centre of the illustration is a plant of 

 Agave Utahensis ; on the left is Echinocactus polycephalus, 

 and in the foreground appears an Opuntia erinacea, Cereus- 

 like in appearance with its narrow, thick, long-spined 

 joints, and very abundant in northern Arizona. The slopes 

 of the cafion are dotted with these and other inhabitants of 

 the desert, but the great Colorado plateau, which is divided 

 by the caiion, is covered with a splendid open forest of the 

 Yellow Pine, Pinus ponderosa, which in northern Arizona 

 and southern Utah forms one of the largest continuous Pine 

 forests of the world. The Pine occasionally extends over 

 the rim of the cafion, and with it other trees and shrubs 

 grow down the sides for a distance of a thousand or fifteen 

 hundred feet ; among them are the White Fir (Abies con- 

 color), the Douglas Fir, the Nut Pine, the Utah Juniper, 

 Cowania Mexicana, the curious simple-leaved Ash (Fraxi- 

 nus anornala), a flowering Ash (Fraxinus cuspidata), the 

 western Shad-bush and Ostrya Knowltoni, one of the rarest 

 and most local of all North American trees, and, up to this 

 time, only known at one place on the slopes of this caiion. 

 It is this mingling of northern and southern plants, the 

 inhabitants of northern mountain forests with the Agaves 

 and Cacti of more southern and more arid regions, which 

 makes the Grand Caiion, which is one of the natural mar- 

 vels of the world and a spot unsurpassed in this country in 

 grandeur and picturesqueness, such an interesting botan- 

 ical ground. 



Agave Utahensis was discovered several years ago by 

 Dr. Edward Palmer near St. George, in southern Utah, and 

 was first described by Dr. Engelmann in Watson's report on 

 the botany of the country along the fortieth parallel of 

 north latitude explored by Clarence King. It is one of the 

 species (Geminiflorse) in which the flowers are borne in 

 pairs in the axil of a bract on a tall scape, and form a dense 

 . elongated spike. The leaves of Agave Utahensis are lanceo- 

 late, concave, thick, hard and glaucous, long-pointed, twelve 

 or fourteen inches long and from two to three inches broad, 

 with pale terminal spines and broad white lateral teeth. 

 The slender spike of bright yellow flowers is three feet 

 long, and is often raised on a stem seven or eight feet high. 

 When the plant is in flower and when the flowers are suc- 

 ceeded by the pale yellow pods which do not fall until late 

 in the autumn or the beginning of winter, and long after 

 the time when they open and shed their seeds, the tall 

 wand-like spike waves backward and forward, and, flicker- 

 ing in the sunlight, enlivens with its strange and wonder- 

 ful beauty the dark forbidding slopes of rock which this 

 plant selects as its favorite home. 



Plant Notes. 



BiLLBERGiA LiBONiAN.^ — During the late summer this is a 

 very showy plant for the conservatory. It is one of the 

 most highly colored of the Billbergias, which, taken to- 

 gether, constitute a most beautiful genus. Twenty or 

 more flowers are borne on a close terminal spike ; they are 

 about two inches long, rich scarlet, tipped with blue ; the 

 petals are more or less recurved, and the sepals half the 

 length of the petals, rosy, fading to Avhite at the base. The 

 anthers are bright yellow, and the stigma blue. The nu- 

 merous bracts below the flowers are lanceolate, two to 

 three inches long, membraneous, a bright ros}' color, the 

 largest crowded immediately below the flower-spike. The 

 scape is twelve to eighteen inches high, white, covered 

 with large rosy bracts. Leaves lingulate, two to three 



