OF LEAVES. 



43 



is to be remarked, that the only irritable ])arts 

 in this leaf are the two or three small glandular 

 points whicli are observed on its upper surl'ace. 

 The movements of plants, unlike those of the 

 higher orders of animals, are effected by organs 

 situated in the part where the movements take 

 place. In the collar of the sensitive plant there 

 is nothing similar to muscles ; yet, while the 

 collar is the scat of motion, as is shown by its 

 incurvations when the leaves are irritated, it is 

 at the same time the seat of mechanism, by 

 which motion is effected. In the sensitive plant 

 there is a long leaf stalk, the base of which 

 swells out into an oval collar, and a similar, but 

 smaller collar, surrounds the base of each pin- 

 nula where it joins the leaf-stalk, and also each 

 leaflet at the point of its .attachment. The centre 

 of the collar is a bundle of vascular tubes, around 

 which is formed a structure consisting of numer- 

 ous oval transparent bags, dispersed through a 

 cellular tissue, with minute corpuscles intermixed. 

 If all the spongy part of the collar be removed 

 from around the central bundle of vessels, the 

 footstalk no longer bends when the leaf is irri- 

 tated ; if the lower half only of the collar be re- 

 moved, the footstalk bends, but cannot be res- 

 tored to the erect position ; when the irritating 

 curve is withdrawn, and if the upper half only 

 be removed, the footstalk remains always un- 

 naturally erect, and does not tend down as usual 

 in the dark, or on the application of external 

 stimuli. Hence the motions of the sensitive 

 leaf are effected by an organic action of the 

 half of the collar, opposite the side towards 

 which incurvation takes place. The movements 

 of the two pinnulse on the leafstalk, and of the 

 several pairs of leaflets in each pinnula, are af- 

 fectedby the same kind of mechanism andactions. 

 This action is in its nature intimately con- 

 nected with the afflux of sap ; for where a proper 

 supply of moisture is withheld, stimuli fail to 

 excite motion, and when a slice of the collar is 

 immersed in water, it immediately bends itself 

 towards what was the side next the centre of 

 the leafstalk. This movement and the whole 

 .iction of the collar depends, according to Dutro- 

 chet, on the cells becoming tinged with sap, ac- 

 cording to his theory of endosmose, which will 

 be explained when treating of the motions of the 

 sap. It may he sufficient to explain here, how- 

 ever, that in all cases where the incurvation of 

 vegetable leaves, or stalks, or tendrils occur, it has 

 been found, that the j)art where the incurvation 

 takes place, is supplied with vesicular corpuscles, 

 as Dutrochet tenns them, diminishing in magni- 

 tude from the convex to the concave side of the 

 curve, the effect of which structure must be, 

 that when the part is supplied with a fluid less 

 dense than the contents of the corpuscles, the 

 larger bodies become most turgid by the flow of 

 sap into them. Incurvation to the opposite side 



must therefore be always produced in such cir- 

 cumstances. There is little difficulty, then, in 

 comprehending how the afflux of sap into the 

 side containing the larger cells, causes incurva- 

 tion of the opposite side inwards ; in fact if a 

 thin slice of it be immersed in water, it imme- 

 diately bends towards the side which was next to 

 the centre of the leafstalk ; and if it be, on the 

 contrary, immersed in a heavier fluid, as strong 

 syrup, it unfolds itself, and bends over to the 

 opposite side. These facts explain sufficiently 

 the mechanism by which the movements of 

 the sensitive plant are produced, but they do 

 not throw any light on the power which exter- 

 nal stimuli have in exciting this mechanism; they 

 do not determine why the flow of the sap causes 

 the incurvation of the upper spring of the collar 

 to predominate over the lower, when the leaf- 

 stalk bends downwards, or why after a little re- 

 pose the lower spring resumes its predominance, 

 and raises the leafstalk. 



Fall of the Leaves. A period arrives every 

 year, when most vegetables are stripped of their 

 leaves. It is commonly at the end of autumn, 

 or the beginning of winter, that trees lose their 

 foliage. But the occui'rence does not take 

 place at the same period in all plants. It is ob- 

 served, in general, that the trees whose leaves 

 are earliest expanded, are also those which lose 

 them first, as is seen to be the case with the 

 lime, the horse-chestnut, &c. The elder forms 

 an exception to this rule ; for its leaves appear 

 at an early season, and are very late in falling. 

 The common ash presents another peculiarity : 

 its leaves are very late in coming out, and fall at 

 the end of summer. 



Petiolate leaves, and especially those which 

 are articulated upon the stem, detach them- 

 selves sooner than those which are sessile, and 

 still more so than those which encircle the stem. 

 In genera], in herbaceous plants, whether annual 

 or perennial, the leaves die along with the stem, 

 without previously separating. Plants which 

 annually shed their leaves are termed Decidvous. 

 On the other hand there are trees and shrubs 

 which remain always adorned with their foliage ; 

 these are the resinous species, such as pines and 

 firs, or certain vegetables whose leaves are stiff 

 and leathery, as the myrtles, alatemi, rose- 

 laurels. These are named Evergreen trees. 



Although the fall of the leaves generally takes 

 place at the approach of winter, cold is not to be 

 considered as the principal cause of this phenome- 

 non. It is much moi-e natural to attribute it to 

 the cessation of vegetation, and the want of 

 nourishment which the leaves experience at that 

 season, when the course of the sap is intei-rupted. 

 The vessels of the leaf contract, dry up, and, 

 soon after, that organ is detached from the twig 

 on which it had been developed. 



The various tints of the autumnal leaf, such as 



