Previous chapters have considered how the basic plan of the vascular plant shoot is initiated and elaborated. It will be recalled that shoot development occurs in two relatively distinct phases. An initial phase involves terminal meristem activity in which the tissues and organs are laid down. There follows a phase of expansion growth in the subapical part of the shoot during which the previously formed structures enlarge and mature. Chapter 11 will examine how variations in the extent of the expansion phase could produce shoots of widely differing morphology. However, there are other developmental variations in the basic body plan of the shoot in which the phase of terminal meristem growth is principally involved, and these are the subject of this chapter.

It should be expected that if terminal meristem activity is modified there might be cases of extreme modification in the kind of organs produced and in the extent and pattern of their subsequent growth and development, and indeed this is so, as any student of plant taxonomy or morphology knows. These modifications have been the subject of extensive researches in which the question has been the degree of homology between the modified organs and more usual organs of the shoot. However, in this chapter attention will be confined to some examples that have proved to be especially amenable to developmental analysis and about which relatively recent information is available.