4.] CHAPTER I. GENERAL MORPHOLOGY. 23 



thus falls off it leaves a more or less permanent scar; the scar on 

 a node which marks the position of a fallen leaf is termed a leaf- 

 scar (see p. 215). 



Hairs and reproductive organs are generally developed as 

 lateral outgrowths upon the members, but occasionally they are 

 developed terminally from the apical growing-point, in some 

 cases directly from the apical cell. They are commonly de- 

 veloped from one or more superficial cells, but in some cases the 

 deeper layers of cells take part in their formation. 



d. The Order of Development of the Lateral Members. Lateral 

 members, with the exception of those which are developed adven- 

 titiously, are developed in a definite order. The rule is that they 

 are developed in such an order that the youngest are nearest to 

 the growing-point, whether the growing-point be apical or inter- 

 calary. This order is termed progressive succession. 



When the growing-point is apical, the youngest lateral members 

 are nearest the apex ; this form of progressive succession is termed 

 acropetal (see Fig. 7 C). 



When the growing-point is intercalary (Fig. 7 1?), the lateral 

 members may be developed on one side, or on both sides, of the 

 growing-point. When the growing-point is near to the base of 

 the parent member, the lateral members are developed above it ; 

 that is, in basipetal succession. When the growing-point is in 

 the middle of the parent member, the lateral members are de- 

 veloped both above and below it. 



Occasionally, particularly in the lower plants and in connexion 

 with the production of the reproductive organs, the law of pro- 

 gressive succession is deviated from by the intercalary develop- 

 ment of members between those already formed. 



4. Arrangement of Lateral Members on a Common 

 Axis. The relative position of members borne on a parent mem- 

 ber, which may be conveniently designated the common axis, may 

 be regarded from two points of view: in relation either to the 

 long axis, or to the surface of the parent member. 



In the former case, similar lateral members may arise singly, 

 at any given level, or several together ; the former is termed the 

 scattered arrangement, the latter the whorled, the group of lateral 

 members at the same level constituting a whorl (Fig. 11). In 

 accordance with the law of progressive development, it may be 

 inferred that the members of a whorl, inasmuch as they are all at 

 the same distance from the growing-point, are all of the same age. 



