164 



MORPHOLOGICAL DEVELOPMENT. 



wise named, furnish conditions under which there is simi- 

 larly an absence of determinate relations between tl 

 parts of the flowers and the incident forces; and hence 

 absence of bilateralness. This inconstancy of reh 



tive position is produced in various other ways — by extreme 

 flexibility of the stems, as in the Blue-bell; by the ten- 

 dency of the peduncles to curl to a greater or less extent 

 in diverse directions, as in Pyrola; by special twistings ol 

 the peduncles, differing in degree in different individuals, 

 as in Convolvulus; by unusual laxity of the petals, as 

 Ly thrum. Elsewhere the like general result arises from 

 progressive change of attitude, as in Myosotis, the stem oi 

 which as it unfolds causes each flower to undergo a transitioi 

 from an upward position of the mouth to a lateral position 

 or as in most Cruciferce, where the like effect follows from 

 altered direction of the peduncle. 



There are, however, certain seemingly-anomalous cases 

 where radial symmetry is maintained by laterally-placed 

 flowers, which keep their parts in relative positions that are 

 tolerably constant. The explanation of these exceptions is 

 not manifest. It is only when we take into account certain 

 incident actions liable to be left unremembered, that we find 

 a probable solution. It will be most convenient to postpone 

 the consideration of these cases until we have reached the 

 general rule to which they are exceptions. 



§ 234. Transitions varying in degree from the radial to- 

 wards the bilateral, are common in flowers that are borne a1 

 the ends of branches or axes which are inclined in tolerabb 

 constant ways. We may see this in sundry garden flowei 

 #33 r^Y such as Petunia, or such 



Isoloma and Achimenes, 

 shown in Figs. 232 and 233. 

 If these plants be examined, 

 it will be perceived that the 

 mode of growth makes the flower unfold in a partially one- 



