550 



ECOLOGY 



chies, one stem circuit making a complete round of the spiral; fig. 729). Progres- 

 sively more complicated arrangements are J, , f, T 5 y , the latter meaning, for 

 example, that there are thirteen orthostichies, and that five stem circuits are 

 necessary for a complete round of the spiral, the fourteenth leaf being above 

 the first, etc. 



Spiral phyllotaxy is advantageous from the standpoint of leaf lighting, since it 

 results in relative remoteness between the members of the same orthostichy; the 

 screw pine (Pandanus) gives an admirable illustration of such arrangement, due to a 

 high-ranked spiral. A relation sometimes is claimed to exist between phyllotaxy 

 and leaf size, that is, complex spirals are supposed to be associated with small leaves 

 and simple spirals with large leaves. Small leaves often occur in many ranks (as 

 in Yucca, Lycopodium, and Polytrichum, figs. 901, 265), and large leaves likewise; 

 probably such relations are fortuitous. The causes of variations in phyllotaxy are 

 not definitely known. A common theory has been that leaf position is due to mechan- 

 ical influences exerted in the bud, such as the pressure of older parts upon those 

 just developing; in recent years, however, many serious objections to this view have 

 been advanced. The \ system that commonly obtains in Phyllocactus is changed 

 to a $ system when the plant is grown in the dark; similar changes have been ob- 



FIG. 789. A mesophytic forest with a luxuriant undergrowth of ferns (Osmunda), 

 whose compound leaves permit the sifting of light and the consequent illumination cf 

 subjacent foliage; Porter, Indiana. Photograph supplied by MEYERS. 



served in Lycopodium and in Schistostega (figs. 787, 788), and in Caulerpa, 

 " leaves " occur only on the lighted side. In any case, no connection need be 

 sought between the causes and the advantages of the various kinds of phyllotaxy. 

 Compound and small leaves. Divided leaves, such as those of ferns (fig. 789), 



