y88 A TEXTBOOK OF THEORETICAL BOTANY 



that each primordium tends to appear in the largest unoccupied space left by 

 its predecessors. This seems to accord not only with observations, but with 

 what we should expect from the conditions of competition for nutriment 

 between primordia, such as must obtain at the apex. It does not exclude the 



Fig. 976. — Plane projection of an apical 

 dome undergoing uniform centrifugal 

 expansion. The growth fields are out- 

 lined by the dotted lines and the 

 positions of successive leaf primordia 

 are represented by circles. (From c/mrc/i, 



" On the Relation of Phyllotaxis to Mechanical 

 Laws," Williams and Norgate , Lid.'). 



possibility that the whole apex may be physiologically mapped out into 

 primordium fields, e.g., by intersecting gradients, even where it appears to 

 be still undifferentiated ; it simply points to the sequence in which these 

 fields will tend to develop. 



Organs of Reduced Foliar Type. 



Every Higher Plant produces various foliar organs which differ from the 

 foliage leaves and are usually spoken of as " reduced," which does not 

 necessarily imply their evolution from foliage leaves, but merely that in 

 comparison with them they have a lower grade of organization. We shall 

 not deal here with floral leaves, but among vegetative structures we may cite 

 as examples : — 



1. Cataphylls : reduced leaves on the lower part of a plant or a shoot, 

 including the cotyledons, the scale leaves on rhizomes, and especially, the 

 bud scales, which mark the base of future shoots. 



2. Hypsophylls : reduced leaves on the upper part of a plant or shoot, 

 especially the bracts in the inflorescence region, and including the prophylls 

 or bracteoles of the individual flowers. 



3. Stipules : foliar appendages attached to the leaf base or the petiole, 

 and including the stipellae which are attached to individual leaflets in a 

 compound leaf. 



