38 



BOTANY 



PART I 



order of arrangement of the rudimentary leaves is determined by their 

 conformity with the position of the older leaves on the vegetative 

 cone, and the consequent necessity of utilising the remaining free 

 space. Thus, the position of newly developing leaves is influenced 

 by those already existing, while their formation is the result of in- 

 ternal causes. After the rudiments of the- new leaves have become 

 protruded from the vegetative cone, they come in direct contact with 

 the older leaves, and may then, as Schwendener has shown, become 

 displaced through the consequent mutual pressure, by which correspond- 

 ing changes in their ultimate position may be effected. If the axis 

 does not grow in length, but only in thickness, as the rudimentary 

 leaves increase in size, their points of insertion will be displaced later- 

 ally by longitudinal pressure ; if the axis increases in length, and not 

 in thickness, the insertion of the leaves will be displaced by a trans- 

 verse pressure. The arrangement of the leaves would also be affected 

 by any increase or decrease in the size of the vegetative cone, un- 

 accompanied by a corresponding increase or cessation of the growth 

 of the rudimentary leaves. Abrupt changes in the usual position 

 of the leaves may also be occasioned by the torsion of their parent 

 stem. Thus, the leaves of Pandanus first appear in three straight 

 rows on the vegetative cone, and their subsequent spiral arrangement, 

 according to Schwendener, results from the torsion of the stem. An 

 irregular arrangement of the leaves, such as occurs, for example, on 

 the flower-stalk of the Crown Imperial (Fritillaria imperialis), may 

 result from the unequal size of the leaves at the time of their in- 

 ception on the vegetative cone. 



A frequent mode of arrangement of foliage leaves is the decussate, 

 in which two-leaved whorls alternate with each other (Fig. 29). A 

 whorled arrangement is characteristic of floral leaves. When the 

 number of leaves in each whorl is the same the 

 '** whorls usually alternate. On the other hand, the 



number of members in the different whorls of 

 floral leaves will often be found to vary greatly ; 

 or a whorl, the existence of which would be 

 expected from the position of other whorls and 

 from a comparison with allied plants, may be 

 altogether wanting. In this connection a com- 

 parison of the flowers of the Liliaceae and Iridaceae 

 will be instructive. The flowers of the Liliaceae 

 (Fig. 40) are composed of five regularly alter- 

 is indicated nating, three-leaved whorls or cycles, viz. a calyx 

 by a Wank dot, oppo- an( j a CO rolla (each consisting of three leaves, and 

 on account of their similar appearance usually 

 referred to conjointly as the perianth), an outer 

 and an inner cycle of stamens, and finally, in the centre of the flower, 

 an ovary of three carpels. In the flowers of the Iridaceae (Fig. 41) 



Fig. 40. — Diagram of a 

 Liliaceous flower. The 

 main axis 

 by a blae 

 site to which' is the 

 bract. 



