254 ANATOMY OF THE LEAF. [BOOK i. 



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 bundle, 

 corresponding 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 node, it 

 will be found that the bundle of woody tissue which forms 

 the frame-work of the leaf communicates above with the 

 medullary 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 autumn, when their existence is drawing to a 

 close. The double layer of fibro-vascular tissue is also per- 

 ceptible in a leaf which has lain 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, 

 and the latter can be easily separated. The curious leaves 

 of Hernias which have the property of opening, upon slight 

 violence, like the leg of a silk stocking, so that the hand may 

 be thrust between their upper and lower surfaces, derive that 

 singular separability from an imperfect union between the 

 layer of excurrent and recurrent fibre. The veins do not, 

 however, always consist of a double layer of fibro-vascular 

 tissue. An instance to the contrary is mentioned by me in 

 the Elements of Botany, as being ' ' furnished by Theophrasta 

 Jussisei. In this plant three layers of veins occur of which the 

 middle is much reticulated, but the upper and lower are far 

 less so, their fibres lying much more parallel with each other, 

 and instead of being applied to the reticulations of the middle 

 layer covering it obliquely." In reality the anatomy of the 

 veins of leaves has been little studied, and must be regarded 

 as a branch of vegetable anatomy, at present not at all well 

 understood. 



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 



