SECT, i MORPHOLOGY 21 



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

 exogenously. 



Buds are formed in the marginal indentations of the fleshy leaves of species 

 of Bryophyllum (Crassulaceae). Although arising from the leaf these buds 

 must properly be regarded as "normal," and as forming part of the normal 

 ontogeny of the plant, since they arise in pre-determined positions from young 

 tissue. In the strict sense of the term only those buds can be called adventitious 

 which are produced in casual positions from tissues which in their production 

 enter into renewed activity, e.g. the buds which arise at the base of isolated leaves 

 of Beyonia when these are laid on damp soil. The concept of " normal b'uds" is 

 notwithstanding usually taken in a narrower sense and limited to buds which 

 arise on the axis of the shoot in the normal course of development. Examples, 

 however, occur which make it difficult to draw a sharp distinction in this latter 

 sense between normal and adventitious shoot-formation ( n ). 



While, as a rule, new leaves arise beneath the apex of a stem, which 

 continues to grow, exceptions to this occur, especially in floral structures ; the 

 apical cone may be used up in the formation of a terminal leaf-rudiment. In this 

 way many of the stamens and carpels of Phanerogams, which are described as 

 terminal, arise. 



In some shoots of limited growth such as the inflorescences of many 

 Phanerogams (Vallisneria) new lateral members arise in an inverted order 

 proceeding from the apex towards the base. The introduction of new lateral 

 members between those already present is also met with in the floral region ( 12 ). 



It was mentioned above that normal shoots arose from the embryonic tissue 

 of the growing point of the parent shoot. When they are apparent at a greater 

 distance from the apex (Fig. 17) it can usually be shown that embryonic substance 

 has been reserved at the proper points for their formation. The growing points of 

 adventitious shoots are for the most part derived from embryonic tissues which 

 have persisted iu the older portions of the plant and are capable of increase. 

 They can, however, also arise from older tissues, owing to the capability of the 

 latter to return to the embryonic condition and produce new growing points. 



Further Development of the Shoot. The processes of develop- 

 ment, which result in the production of new members at the apex of 

 a shoot, are followed by their increase in size and further growth. 

 This growth is usually introduced by the vigorous elongation of 

 the segments, which is spoken of as growth in length, while at the 

 same time the buds expand. The zone of most rapid growth in 

 length is at some distance from the growing point. The growth 

 in length and consequent elongation of the shoot is in some cases 

 so slight that the leaves remain close together, and leave no free 

 spaces on the stem, thus forming so-called SHORT SHOOTS. The 

 same plant may bear ordinary elongated shoots and short shoots. 

 The duration of life of the latter is usually shorter, and they fre- 

 quently do not branch, and take no part in the permanent branch 

 system of the tree. This is seen in the Larch, in which the 

 short shoots with crowded needle-shaped leaves are borne on 

 elongated shoots. The stem of a shoot, as contrasted with the leaves, 



