22 STRUCTURE OF PLANTS. 



point, it can no longer be distended, and the diameter ceases to increase. 

 Hence, generally, the life of an endogenous tree seems more limited than that 

 of an exogen ; because it is well known that trees of the latter kind will 

 live for an indefinite period, and even for centuries, after the interior of the 

 trunks have become entirely rotten, and their circumference separated so as 

 to form vertical sections, or fragments of trunks, with rotten wood on one 

 side, and living bark and growing shoots on the other ; the increase both of 

 bark and wood still going on. Endogens differ from exogens in commonly 

 developing only a terminal bud, as in Palms, in which case the stem is of 

 the same thickness throughout, and cylindrical ; but when several buds deve- 

 lope themselves, as in the stem of the Asparagus, and in that of the Bamboo, 

 the stem becomes conical like the stems of exogens. 



88. Though the normal direction of stems and branches is upwards, or 

 at all events, above the surface of the ground, yet there are exceptions in the 

 case of creeping roots, as in the Everlasting Pea ; in rhizomas, which are un- 

 derground stems, as in the water-lily, and the common reed ; in tubers 

 which are stems under the surface, as in the potato ; and in corms, as in the 

 crocus, the root of which, though commonly called a bulb, is, botanically, a 

 dilated stem. 



89. JVodiy or knots, are the places where buds are formed, and internodi 

 the spaces between them. Whatever is produced by a leaf bud is a branch, 

 which, when in a growing state, is called a shoot. Leaf-buds sometimes are 

 imperfectly developed so as to form a spine, with or without leaves, as in the 

 common hawthorn; and such spines are therefore imperfectly developed 

 branches. All growths from the stems which are not the evolutions of leaf 

 buds, as for example the prickles, are modifications of the cellular matter, 

 and of the epidermis of the bark. The uses of prickles to the plant appear 

 imperfectly understood. 



90. Buds are either leaf-buds or flower-buds, and the former are either 

 regular or adventitious. Regular leaf-buds are only found in the axils of 

 the leaves, or in the axils of their modifications. Hence as scales, stipules, 

 bracts, sepals, petals, stamens, and carpellas, are considered as metamor- 

 phosed leaves, adventitious buds are believed to exist on their axils ; though 

 they are rarely developed in a state of nature and only sometimes by artifi- 

 cial processes. Regular buds alone develope themselves untouched by art 

 or accident ; and hence, whatever may be the arrangement of the buds, 

 the same will be that of the branches. Adventitious leaf-buds are found 

 surrounding the bases of regular leaf-buds, and in general where there is 

 an anastomosis of woody fibre. They are found in the roots of a number 

 of plants, and sometimes on the margin of leaves, or at the base of their 

 petioles ; they are never visible either on the root or stem till they begin to 

 develope themselves and burst through the bark. 



91. Leaves are expansions of the bark, and only found at the nodi of the 

 stem. They are developed as the stem advances in growth, one above and 

 after another, opposite, alternate, or verticillate, and in each of these modes 

 with greater or less regularity. A complete leaf consists of a petiole or foot- 

 stalk, a lamina or disk, and a pair of stipula or small side leaves at the base 

 of the petiole. The lamina is sometimes wanting or changed in shape, and 

 sometimes the petiole is extended, and instead of terminating in a lamina, it 

 assumes a cylindrical wirelike figure, and becomes a tendril. The veins of 

 leaves branch in all exogenous plants, with the exception of the Coniferae and 

 Cycadeae orders, the stems of which have an exogenous structure, while the 



