RADIAL SHOOTS. MECHANICAL HYPOTHESIS OF LEAF-POSITION 77 



the same. In nature, however, markedly obliquely-placed organs occur relatively 

 seldom. 



Few organs retain during the whole course of their development the same 

 form. Most of them show considerable changes which are partly active, brought 

 about by growth, partly passive, flattenings caused by reciprocal pressure. In 

 many cases these attain to such a pitch that the circular transverse sections of 

 the primordia become polygonal at an early period, and they lie touching one 

 another without interspaces. The cones of many Coniferae, the heads of Com- 

 positae, the fruit of the pine-apple, furnish examples. In these cases there 

 is contact of the organs in three directions usually during a long period, and we 

 have then in a certain degree to deal with a span-roof with three rafters. As 

 Schwendener has shown, the lateral displacements suffer then generally a diminu- 

 tion ; the approximation to the limiting value takes place with oscillations of 

 small width. 



Having in what precedes dealt with the displacements which a given arrange- 

 ment of lateral organs experiences in course of the development, let us now turn 

 our attention to the mode of installation of the primary positions. Observations 

 of the apex of shoots teach us that new organs are always laid down upon them 

 in connexion with those which precede them in acropetal serial succession. 

 Hofmeister ! was the first who endeavoured to give a mechanical explanation of 

 this fact in his dictum that new organs arise in the widest intervals that occur 

 between the organs already existing. Whilst this statement cannot be accepted 

 now in the form in which it was made, yet we must acknowledge that Hofmeister 

 rightly recognized that the position of new organs is conditioned by that of the 

 older ones. Schwendener has carefully investigated these relationships in many 

 examples, and he has shown it to be a general rule that the young organs are 

 laid down in contact ivith the older. In order to avoid misunderstanding it is 

 necessary to state specifically that this contact of the young papillae of the leaves 

 generally takes place only in two parastichies, and that there is no contact as 

 a rule of the orthostichies. Of course one cannot speak of a literal contact of the 

 primordia until they appear upon the surface of the mother-organ, yet as a matter of 

 fact the youngest stages of lateral organs can be recognized through the microscope 

 at a much earlier period ; their centres of formation can be seen at certain dis- 

 tances from one another so that every primordium occupies a definite developmental 

 field which it completely fills up in the course of its construction but cannot 

 overstep because the adjacent primordia claim completely the areas that in like 

 manner belong to them 2 . That these areas of development have a definite size, 

 and so long as the organs are similar an almost constant size, is a morpho- 

 logical fact requiring no further explanation, no more than does that of the mutual 

 limitation of these areas and the contact of the juvenile organs which results from 

 this. The relative size of the primordia and their contact with preceding organs 



1 Hofmeister, Allgemeine Morphologic der Gewachse. Handbuch der physiologischen Botanik, i. 

 Abt. 2. Leipzig, 1868. 



2 Schwendener, Die jiingsten Entwickelungsstadien seitlicher Organe und ihr Anschluss an bereits 

 vorhandene, in Sitznngsber. d. Berliner Akademie d. Wisscnsch., 1895, p. 645. 



