20 BOTANY part i 



Although in the vegetative regions, i.e. the regions in which merely 

 vegetative organs are produced, the rudiments of the new shoots of 

 phanerogamic plants make their appearance much later than those of 

 the leaves, in the generative or flower-producing regions the forma- 

 tion of the shoots follows directly upon that of their subtending leaves, 

 or it may even precede them. In this last case the subtending leaves 

 are usually either poorly developed or completely suppressed, as in the 

 inflorescence of the Crutiferae, in which a series of phylogenetic changes 

 has probably led to this result. 



Shoots developing in definite succession from the growing points of 

 other shoots are designated normal, in contrast to adventitious 

 SHOOTS, which are produced irregularly from the older portions of a 

 plant. Such adventitious shoots show no definite arrangement, and 

 frequently spring from old stems, also from the roots of herbaceous 

 plants (Brassica oleracea, Anemone sylvestris, Convolvulus arvensis, Eumex 

 Acetosella), or of bushes (Bubus, Bosa, Corylus), or of trees (Populvs, 

 Ulmus, Bobinia), or they may develop even from leaves, particularly 

 from the fronds of Perns. An injury to a plant will frequently induce 

 the formation of adventitious snoots, and for this reason gardeners 

 often make use of pieces of stems, rhizomes, or even leaves as 

 cuttings from which to produce new plants. A leaf of a Begonia 

 merely placed upon damp soil will soon give rise adventitiously to 

 new plants. 



Leaves and also normal shoots, which make their appearance as 

 outgrowths from the portions of the parent shoot still in embryonic 

 condition, have an external or exogenous origin. Adventitious shoots, 

 on the other hand, which arise from the older parts of stems or roots, 

 are almost always endogenous. They must penetrate the outer 

 portions of their parent shoot before becoming visible. Adventi- 

 tious shoots formed on leaves, however, arise, like normal shoots, 

 exogenously. 



The further Development of the Shoot. — All normal shoots 

 are dependent for their origination upon the embryonic substance 

 of the growing point of the parent shoot ; even when they make 

 their appearance at some distance from the growing apex (Fig. 17), 

 embryonic substance has been reserved at that point for their forma- 

 tion. The growing points of adventitious shoots are also, for the most 

 part, produced from tissue which has retained its embryonic condi- 

 tion in the older portions of the plant. In some cases, however, they 

 arise from newly-developed growing points, and afford evidence of 

 the power inherent in plants to return to an embryonic state and 

 produce new growing points. The processes of development which 

 result in the production of new segments at the apex of a shoot are 

 followed by an increase in size and by the further growth of the 

 segments. This growth is usually introduced by the vigorous elonga- 

 tion of the segments, by means of which their rapid unfolding from 



