REJUVENESCENCE IN NATURE. 63 



rare instances they are delicately membranous, in which 

 cases the axis which bears them is mostly fleshy ; their 

 colour is never a decided green, generally whitish, passing 

 into yellow, flesh-colour, brownish, or even black. Their 

 development goes on very slowly; they are tolerably en- 

 during, and the chief part of their existence is passed in 

 the winter season. 



2. EupUyllary formation (Laub -blatter}. These are 

 the organs, especially and ordinarily simply called "leaves," 

 which give most character to the vegetative structure or 

 stock. They are readily distinguished from the leaves of 

 the preceding formation by the greater longitudinal 

 development with a narrower base, in general more con- 

 siderable dimensions, and the green colour, never absent 

 although in many cases concealed. Their especial mark 

 is the blade-structure, with which is ordinarily combined 

 its contrary, the stalk or petiole structure. Through 

 multifold alternations of the conditions of expansion and 

 contraction arises the so frequent production of divided 

 and compound euphyllary-leaves, to which also belongs, 

 as a special case, the division into main- leaf and accessory 

 leaves (stipules). The multiplicity of conditions of ner- 

 vation within the body of the blade, corresponds to the 

 multiformity in the outline of the leaf. The consistence 

 is mostly stoutly membranous, frequently leathery, more 

 rarely fleshy. The principal part of the life is passed in 

 the summer ; the duration is considerable, especially in 

 those of fleshy or leathery consistence. 



3. Hypsopliyllary formation (Hocli-Uatter}, to which 

 belong the involucral leaves and common calyces of inflo- 

 rescences, bracts and bracteoles, glumes and palea3, which 

 accompany the flower. These again approach in charac- 

 ter the cataphyllary leaves, as the stalk and blade-struc- 

 tures, as well as the green colour, vanish more or less or 

 even quite completely.* They are distinguished from 



* Stalked hypsophyllary-leaves are a rarity, e. ff., in Podolepis; the 

 formation of a stalk occurs more frequently above the sheath-like part of the 

 leaf, as for instance, in the formation of awns on the glumes of the grasses. 



