296 ANATOMY OF LEAVES. 



impunity. The necessity of such a modification of 

 structure, in the petioles of compound leaves, sus- 

 ceptible of motion, may indeed be inferred from the 

 fact, that articulations are present in all those which 

 perform certain movements ; as, for example, those 

 which fold together their leaflets at night ; those which 

 are endowed with the power of spontaneously moving 

 their leaflets, as Hedysarum gyrans ; and those which 

 fold their leaflets together when touched, as Mimosa 

 sensitiva and pudica, &c. 



Some simple leaves, as those of the Hollyhock, of 

 the Geranium tribe, &tc. which have several principal 

 costae diverging from the summit of the petiole, and in 

 this respect allied to digitate leaves, present nearly 

 the same vascular structure of the petiole as the com- 

 pound leaves. The fasciculi are distinct, and corres- 

 pond in number to the principal costas of the leaf; 

 each of which may be thus regarded as a kind of mid- 

 rib, and the leaf as composed of a number of con- 

 joined leaflets ; so that these leaves, although they 

 are necessarily classed as simple leaves from their 

 external appearance, yet, bear in anatomical structure 

 the same affinity to digitate compound leaves, which 

 the webbed foot of a bird bears to one which is not 

 webbed. A similar structure, also, is found in the pe- 

 tioles of those leaves which are longitudinally ribbed, 

 or nerved, as the common expression is, from the base 

 of the expansion, as, for instance, those of the genus 

 Melastoma ; but, when the ribs do not originate from 

 the base, although they are very conspicuous, as in 

 the leaf of the Cinnamon tree, the structure of the 

 vascular system of the petiole is exactly the same as 

 in simple dicotyledonous leaves, which are not longi- 

 tudinally ribbed. 



If, instead of a transverse section, we place a lon- 

 gitudinal section of any of these leaves under the mi- 



