GENEHAL PHYSIOLOGICAL CONSIDEBATIONS. 545 



duration, which annually sends up a flowering shoot or shoots. Other 

 perennial plants of this kind form one shoot, which vegetates uninter- 

 ruptedly for many years before it flowers (Agave americana, Talipot 

 Palm, &c.) ; and after ripening its seeds the stem dies down, leaving 

 usually a number of offsets from the axils of its leaves (monocarpic peren- 

 nials}. 



Woody perennials, trees and shrubs, usually vegetate for several years 

 before flowering, but are subject to periodic rest, throwing off their 

 foliage and renewing it upon fresh shoots of the same stem every season ; 

 and when they flower, the operation exhausts their accumulated powers 

 of development so little that they continue to flower periodically (every 

 season if in favourable condition) throughout life. 



Habit of Plants. The " habit " assumed by plants depends in some 

 degree on external conditions. Thus many of our garden annuals are 

 perennial in their native climates : for example, Eicinus (the Castor-oil 

 plant), Mirabilis, and other genera are annual herbs with us, but perennial 

 and even woody in warmer climates. And some annuals may be made to 

 vegetate for more than one season by removing the flower-buds as they 

 appear, as in the case of the so-called Tree-rnigiionnette. The Winter- 

 corn of agriculturalists is really an annual plant, sown in autumn to 

 obtain stronger growth, and is not specifically difl'erent from Spring- 

 corn, sown in spring and reaped in autumn. The common Cherry-tree 

 retains its leaves during the whole year and becomes an evergreen in 

 Cevlon; and many similar instances of changed habit, the result of 

 altered condition, might be cited, while for further particulars respecting 

 the duration of plants the student may refer to the sections treating 

 of the Morphology of Stems. 



Few perennial plants retain their appendicular organs beyond certain 

 definite periods. Ordinary deciduous trees lose their leaves in autumn in 

 our climate ; and previously to their fall their organs undergo internal 

 changes, in which the assimilated matters are, for the most part, removed 

 and their green colour altered. They are generally cast off by a regular 

 fracture where they join the stem (p. 540) j in the Oak, Beech, and other 

 trees they die in autumn, but do not fall away at once, often remaining, 

 when not exposed to violent winds, until pushed off by the expansion of 

 the stem in the next spring. Evergreen trees and shrubs retain their 

 leaves green and living until the succeeding season, when the new leaf- 

 buds expand, as in the Cherry-laurel, Aucuba, &c. j or, as in many Coni- 

 ferse, they remain attached to the stem for several years (Araucaria 

 imbricata, Thuja, &c.). In some of these cases the so-called leaves are 

 probably f oliaceous branches. The leaves of arborescent Monocotvledons 

 (Palms) are also of long duration. The parts of flowers and ripe fruits 

 are likewise cast off in most cases, although the fruits from which 

 seeds have escaped sometimes remain long attached in a dead condition 

 (Conifers). 



The axis is the only permanent part of the plant ; and the unlimited 

 duration of this is strictly dependent on the development of leaf-buds. 

 When a shoot ends in a blossom-bud, the growth of that branch of the 

 axis is arrested, and the prolongation of life depends either on the axillary 

 leaf-buds situated below or on the formation of an adventitious bud. 



The production of flowers and fruit is an exhausting process j it has 



