600 PHYSIOLOGY. 



Sometimes the two modes of leaf -formation above described coexist in 

 the same leaf ; that is to say, the lobes of a leaf may be formed from above 

 downwards, while the nerve passing into each of them gives off its branches 

 from below upwards. 



The opposition, alternation, or spiral arrangement of organs 

 depends on the period at which they are developed : thus, if two 

 or more leaf-organs be developed at the same time and in equal 

 degree, a whorl is produced ; if the development be successive, not 

 simultaneous, the organs are then arranged alternately or spirally. 



Development of the Parts of the Flower. The evolution of the 

 parts of the flower takes place after the same general fashion as 

 that just mentioned in the case of leaf-organs in general. The 

 causes producing irregularity and deviations from the typical floral 

 symmetry have been already alluded to (p. 91). It should, how- 

 ever, be remembered that those irregularities are often congenital, 

 ?". e. exist from the very beginning ; in other cases, the symmetry is 

 perfect at first, but becomes subsequently irregular. The con- 

 genital arrangement not only shows the earliest stage of the indi- 

 vidual flower, but it gives an indication of the ancestral condition. 

 A flower becoming irregular in course of growth, suggests that the 

 primitive type from which the flower in question has descended 

 was regular, the irregularity accruing from adaptation to particular 

 conditions during growth or to special requirements. 



Calyx. If the sepals are indefinite they originate spirally; if definite 

 they are either equal or unequal in number. If equal, the members of 

 each pair originate simultaneously, the lowermost pair first. In a 

 pentamerous calyx, developed on the J plan, it is assumed by Payer 

 that one part of an originally quaternary flower is divided into two. Thus, 

 if there are two bracteoles, as previously explained, and two pairs of de- 

 cussating sepals, the anterior sepal of the lower pair (see p. 89) becomes 

 divided, and one of the two subdivisions is lower than the undivided 

 posterior sepal. This lowest sepal is No. 1 ; the other subdivision 

 is placed a little higher than the posterior sepal, and is No. 3; 

 the posterior undivided sepal of the lowermost pair is thus placed 

 between 1 and 3, and is therefore No. 2. The two sepals of the upper 

 pair are 4 and 5 respectively. The sepals 1, 2, 3 belong thus to the 

 lower series, 4 and 5 to an upper one. If, however, there are either no 

 bracteoles or two pairs of bracteoles and an originally quaternary calyx, 

 it is the upper pair of sepals which is antero-posterior, as in Epi- 

 lobittm. The anterior sepal divides as before, to form Nos. 3 and 5, the 

 posterior sepal being No. 4, and the two lower lateral sepals 1 and 2 re- 

 spectively. There is, however, no trace of splitting, and it must be 

 assumed to be hypothetical. The development of the -| arrangement 

 from the formation of internodes between primitively decussate and oppo- 

 site leaves, as explained by Henslow, seems the more probable explana- 

 tion (p. 48). 



