98 ORGANOGRAPHY. BOOK I. 



issues, the cellular tissue of the bark also diverges, accom- 

 panying the fibro- vascular tissue, expanding with its ramifi- 

 cations, and filling up their interstices. The tissue that 

 proceeds from the medullary sheath, after having passed from 

 the origin of the leaf to its extremity, doubles back upon 

 itself, forming underneath the first a new layer of fibre, 

 which, upon its return, converges just as the first layer 

 diverged, at length combining into a single bmidle, corre- 

 sponding in bulk and position to that which first emerged, 

 and finally discharging itself into the liber. If, therefore, a 

 section of the leaf and stem be carefully made at a nodus, it 

 will be found that the bundle of woody tissue which forms the 

 frame-work of the leaf communicates above with the medidlary 

 sheath, and below with the liber. This is easily seen in the 

 spring, when the leaves are young ; but is not so visible in 

 the autumji, when their existence is drawing to a close. The 

 double layer of fibrovascular tissue is also perceptible in a leaf 

 which has laid during the winter in some damp ditch, where 

 its cellular substance has decayed, so that the cohesion between 

 the upper and lower layers is destroyed : they can then be 

 easily separated. The curious Indian leaves which have the 

 property of opening, upon slight violence, like the leg of 

 a silk stocking, so that the hand may be thi-ust between 

 their upper and lower surfaces, derive that singular separa- 

 bility from an imperfect union between the layer of excurrent 

 and recurrent fibre. De Candolle remarks, that, when the 

 fibres expand to form the limb of a leaf, they may (whether 

 this phenomenon occurs at the extremity of a petiole, or at 

 the point of separation from the stem) do so after two different 

 systems : they may either constantly preserve the same plane, 

 when common flat leaves are formed ; or they may expand in 

 any direction, when cylindrical or swollen or triangular leaves 

 are the result. {Organogr. p. 270.) 



The cellular tissue of which the rest of the leaf is composed 

 is parenchyma, which Link then calls diachyma^ or that im- 

 mediately beneath the two surfaces cortex^ and the intermediate 

 substance diploe. De Candolle calls these two, taken to- 

 gether, the mesophyllum. The whole is protected, in leaves 

 exposed to air, by a coating of cuticle, furnished with stoma- 



A 



