58 



ANALYTICAL CLASS-BOOK OF BOTANY. 



off inert or dead matter, with a force proportioned to 

 their degree of vitality ; or, in other words, Life, being 

 the positive and active principle, continually asserts its 

 power, by rejecting dead substances which are only pas- 

 sive and negative. Thus dead flesh is thrown off by the 

 animal body, in the ulceration of a sore ; diseased bones 

 are rejected, and dead hair and teeth are cast off, in the 

 vital action of a new growth. Keeping this principle in 

 view, you will be better able to understand the exciting 

 cause of falling in the leaf, which is often only a kind of 

 sloughing, or casting off of worn-out garments. 



319. Early in the season, often in the freshness of 

 the spring-tide leaf, there is a faint line to be traced 

 round the base of the petiole. This is caused by the 

 formation of a joint between the base of the leaf-stalk 

 and the stem, or branch, on which it stands ; and as the 

 season advances, it becomes more strongly marked. In 

 a transverse layer of cells which follows this line, the 

 substance becomes decomposed by the vital action of the 

 forming articulation, which the Leaf, with its diminishing 

 vitality, cannot resist. And thus cell by cell it is cut 

 off, and finally drops to the ground. This may be seen 

 by examining the petiole of fallen leaves, which often 

 appear to be cut off as smoothly as if the operation were 

 performed with a sharp knife. 



320. That the fall of the leaf is not caused by frost, 

 nor by the actual death of the leaf, and also that when 

 the leaf dies it will not necessarily fall, may be shown 

 by several facts. In the great Plane or Buttonwood tree, 

 the bud of the next year is formed within the leaf-stalk 

 of the present ; and thus the fall of the leaf is caused 

 directly by its protrusion. The leaves of some species 

 often turn red, and sometimes fall, before the appearance 

 of frost ; and when young leaves are killed by frost in 

 spring, they do not fall, but decay and wither on the 

 trees, because there are then no articulations formed, and 

 no protruding buds to effect a separation. Palms, and 

 most Endogens, never exclude their old leaves, because 

 they are not articulated with the stem, as in those of 

 Exogens; and their remains continue hanging about the 

 tree long after the appearance of the new leaves, or until 

 they are corroded by the elements, and fall away by 

 atoms. This is also, in some degree, true of the Beech 

 and Oak, their leaves often remaining through the win- 

 ter, and until they are pushed off by the expanding buds 

 in the spring. 



CHAPTER XXII. 



THE FOOD OF PLANTS. 



321. THE elementary constituents of plants are of 

 two kinds, as you have already learned ; namely, Orga- 

 nic and Inorganic. Carbon, hydrogen, oxygen and nitrogen, 

 which are termed the universal organic constituents of 

 plants, because they enter more or less into all organism, 

 compose the first ; and certain earthy or mineral matters, 

 the second. The completed tissue is made up of the three 

 first, nitrogen, or , as it is sometimes called, azote, being intro- 

 duced only as an agent in the vital action of the cells, or as a 

 deposit in their granaries, but never fixed in the structure. 

 As these four elements are universal, it follows that the 

 distinguishing characteristics of plants must be traced to 

 the inorganic elements which enter into their composition ; 

 and so it is. 



322. COMMON SOURCES OF FOOD. Of the four ele 

 ments which enter into the composition and vital move- 

 ments of all plants, carbon constitutes from fifty to sixty 

 per cent, of the whole structure. This enormous amount, 

 which thus supplies and saturates all vegetative nature, is 

 drawn, either directly or indirectly, from the atmosphere. 

 Oxygen and hydrogen are furnished by rain-water ; and 

 nitrogen is obtained chiefly in the form of ammonia. 



323. PARTICULAR SOURCES OF FOOD. But while the 

 organic elements have always maintained their due con- 

 sequence among scientific Botanists and Cultivators, the 

 importance of the Inorganic elements has not been ap- 

 preciated. These are chiefly composed of Potash, Soda, 

 Lime, Phosphoric Acid, Sulphuric Acid, Magnesia and 

 Silica, or sand. These, and some others, may be termed 

 the particular, or INDIVIDUAL CONSTITUENTS of plants ; 

 and they are drawn from the soil. To determine of what 

 kind, degree, and in what proportion, the inorganic ele- 

 ments are combined in any plant, we have only to ana- 

 lyze the ashes which it deposits in burning. Thus we 

 arrive at certain conclusions, in regard to the kind and 

 degree of food that should be supplied ; for the inorganic 

 elements which, in a healthy or natural state, are taken 

 up into the tissues, should not only be present in the 

 soil to which the seed of any species is consigned, but 

 they should be present in precisely the same proportions 

 they have in the healthy plant. 



324. Liebig, the great German Chemist, has made 

 many experiments in these mineral constituents of the 



Instances. What may the fall of the leaf often be termed ? How is the 

 leaf detached ? Describe the operation. What facts show that the fall of the 

 leaf is not caused by frost, nor by death ? How does the Plane reject its 

 leaves ? Why do not Palms reject theirs ? How are the Beech and Oak? 



General subject Elementary constituents of how many kinds ? Name 



the Organic elements which are deposited in the structure which is the vital 

 agent ? What proportion of carbon from what source oxygen and hydrogen- 

 nitrogen ? What class of elements have been neglected ? Of what are these 

 chiefly composed what termed what analysis for ivliat purpose what re- 

 sult? What should be present in the soil? What great experimenter ? 



