162 ON THE TORPIDITY OF PLANTS. 



rupeds, and even the hair of the head of man himself; thus we see 

 in the animal creation an universal tendency to moult. No less in- 

 controvertible is the fact with respect to the vegetable creation ; hence 

 there must be a grand and universal cause operating upon all nature, 

 and this cause, no doubt, is the change of seasons. 



But it will be our duty to examine a little more minutely into the 

 physiological operations causing this vegetable moult. 



The effect, which is the most evident, is the fall of the leaf, or 

 defoliation of the plant. As a general rule this commences with the 

 decrease in temperature of autumn, and increases with the frost of 

 winter, although it may not hold good with respect to the plants or 

 shrubs called evergreens: nevertheless, there is even in these, an annual 

 moult, though not so evident as in the forest and garden trees, as 

 in them the fall of leaf most generally occurs in spring, when the 

 young leaves have began to appear; yet in warm climates there 

 are many plants that retain their leaves even for several years. 



To understand the present subject perfectly, it will be necessary 

 that we should be acquainted with the anatomical connection the leaf 

 has with the plant. To do this, it will be sufficient to know that, from 

 the sheath which surrounds the pith, a bundle of woody tissue is 

 emerged, accompanied by what are called spiral vessels, from their 

 assuming the form of a cork-screw; which fact is easily proved by 

 making a circular cut through the superficial covering of a new shoot 

 or foot-stalk of a leaf of the vine, and by gently drawing the 

 cut part asunder, the minute spiral threads will be made evident, 

 even to a careless observer. This fibre- vascular bundle passes through 

 the bark, and proceeds at an angle more or less acute to a detennined 

 distance from the stem, branching oft at intervals, and by numerous 

 ramifications forming a kind of net-work. This net-work is filled up 

 by cellular tissue, which gives solidity to the leaf, the cellular tissue 

 being continuous with that of the bark. After the fibro-vascular tissue 

 has passed from the origin of the leaf to its circumference, it turns 

 upon itself, fonning a second layer of fibre under that already men- 

 tioned, which gradually converges just as the fonner has diverged, and 

 ultimately is fonned into a single bundle, the same size as that which 

 emerged, and is finally discharged into the liber, or inner bark. 



Thus we perceive that the leaf is continuous with the medullary or 

 pith sheath by its superior layer of fibres, and with the liber or 



