90 



we that the accepted good species are not thus derived from actually living or from anterior types? 

 This is only an hypothesis; but the natural polymorphism that we observe in the similar elements of 

 the same plant may well represent, when fixed upon that plant, the variations which, in other circum- 

 stances, would become detached and isolated, and live separately, protected by the generation of a 

 certain fixity. 



We do not pretend to resolve this complex problem of the species in this manner; but we find here 

 an argument in favour of the general theory of derivation, opposed to the theory of the absolute fixity 

 of the types of successive creations by a sort of repeated miracle. (These were looked upon as advanced 

 opinions half a century ago, J.H.M.) Let us, however, leave these nebulous regions of philosophic specula- 

 tion and descend to the facts concerning the Eucalyptus globulus. ... (J. E. Planchon's article on 

 E. globulus in " Revue de Deux Mondes," 1875, translation by United States Department of Agriculture, 

 pp. 8, 9.) 



Casimir de Candolle., 1903. — Adventitious buds are those which arise accidentally in indeterminate 

 points of the body of the plant, or on parts of it which do not normally produce them. Their disposition 

 is therefore irregular; that is to say, without relation to that of the leaves and of the ordinary branches. 

 They must not be confused with certain axillary buds whose retarded evolution only takes place a long 

 time after the falling of the leaves in the axil- of which they have taken birth, and which are called, for that 

 reason, dormant buds. 



Adventitious buds are met with very often on the trunks or on the branches of trees, and more 

 rarely on herbaceous stems. Many plants produce them also on their roots or their leaves. They are 

 even formed sometimes in the interior of seeds, where they constitute adventitious embroyos. 



I am only here concerned with those which are produced on the trunk and on the branches of trees 

 and shrubs. They are always 1 of endogenous origin, arising in the tissue around the cambium. The 

 cellular layer in which the formation of these buds begins has only yet been determined absolutely in a 

 small number of dicotyledons, 2 in which it is found to be the pericycle, and everything leads to the belief 

 that it is the same with other plants of this class. 



The shoots issuing from the adventitious buds always exhibit, at the beginning, vegetative 

 characteristics of the young plant of the same species. Also they are never entirely similar to those of 

 the axillary buds of the adult tree. In certain cases they differ from them even in a striking manner. 

 This pecularity of the adventitious shoots deserves to be closely examined, and it is this that I propose 

 to do in the following pages. 



I shall commence by recalling that every plant begins, in its development, by the production of 

 phyllomes belonging to the simplest types, such as the cotyledons and the basilar scales which succeed 

 them on the primary stem of many of the species-. Then come the true leaves, of which the dimensions 

 and also the degree of complication increase from one to the other, more or less rapidly, until there is 

 attained the t3'pe of the definite leaves characterising the species to which they belong. One may then 

 say that every plant presents, in the course of its individual evolution, a more or less varied heterophylly. 

 There are produced sometimes, at the same time, changes in the form and the structure of the successive 

 axes. Finally, the phyllotaxy even of the leaves may also change, and it is then.the lower leaves which 

 present arrangements the most condensed in character. 



The different phases of individual evolution succeed each other generally quick enough, because 

 many plants have acquired already on their primary stem their adult leaves. In this case the latter differ 

 from those which come later on the adult plant. They are distinguished, however, almost always by a 

 certain juvenile appearance belonging to characters it is difficult to define, such as the slight differences 

 of form, of consistence, or of colouring. This same juvenile appearance is always found in the adventitious 

 shoots of the adult plant. There are also species in which the young plant has very similar leaves, as to 

 form and dimensions, to those of the adult plant, but having a simpler internal structure. The first leaves 

 of the adventitious shoots always display equally this same simplicity of internal structure. 



1 But see Vines' "Students' Text-book of Botany." — "They may be produced endogenously or exogenously 

 (1) from a single epidermal cell, (2) from epidermis and pericycle." 



2 Also Cuiciita and ConvoCvwlvs. See Vines' " Students' Text-book of Botany," p. 190 — " When the part is very 

 young the adventitious member is developed exogenously ; when the part is older the adventitious member is developed 

 endogenously, usually from the pericycle, but sometimes from still deeper tissues." 



