1 1 68 A TEXTBOOK OF THEORETICAL BOTANY 



parastichies. In flowers with thirteen parastichies, five will begin with 

 stamens which stand approximately alternate to the perianth members. 

 With reduction in the size of flowers and a consequent reduction in the 

 number of stamens, due to lack of space, it appears to be these alternating 

 stamens, and those in their parastichies, which have been retained, while 

 the intervening ones disappeared, thus completing the disruption of the 

 primitive genetic spiral and leaving isolated parastichies in which the 

 corresponding stamen members will appear to be at the same level and will 

 appear simultaneously at the growing point. In other words they will form 

 a series of whorls and the flower will now be cyclic. The cyclic arrange- 

 ment, with stamens either limited to one whorl consisting of the first 

 members of each parastichy or alternating in two successive whorls, is 

 more adaptable to the conditions of close spacing, especially in smaller 

 flowers, than the spiral or hemicyclic arrangements and has remained a 

 constant feature of all the higher families. 



The chief exception to the general rule of alternation between the outer 

 stamens and the petals arises in the case of obdiplostemony, where they 

 stand opposite the petals. The probable reasons for this anomaly we have 

 already sufliciently discussed on page 1095. The case oi Potamogeton, where 

 the petals arise directly from the backs of the stamens, may be otherwise 

 accounted for, as here we seem to have before us petaloid outgrowths of the 

 stamens, and not true petals. 



The small flowers of the monocotyledonous genus Triglochin present a 

 noteworthy peculiarity (see Fig. 107 1). The three stamens of the outer 

 whorl stand opposite to the three outer perianth members. These are 

 followed by a second whorl of three perianth members, alternating with the 

 outer stamens, and to these perianth members succeed the three inner 

 stamens which are also opposite to them. Not only is there a correspondence 

 in position but there is also an organic connection between the stamens and 

 their opposed perianth segments shown by their falling together as units. 

 Flowers of Triglochin occasionally occur which are dimerous, not trimerous, 

 with four stamens and four perianth parts in pairs, instead of six in threes, 

 and these flowers so closely resemble the dimerous flowers of Potamogeton 

 that we may conclude that the same explanation is applicable here, namely 

 that the apparent perianth segments are really outgrowths of the stamens 

 themselves and that there is no true perianth present. In the related genus 

 Ruppia, which is also dimerous, the apparent perianth parts are reduced to 

 mere scales on the backs of the stamens and the flower is naked. 



A somewhat similar position exists in several other families. The 

 flowers of the Proteaceae, for example, are usually tetramerous, with a 

 perianth composed only of one whorl of four sepals, to which are adherent 

 four opposite stamens, only the anthers being free and seeming to arise near 

 the tips of the sepals, which often cover them with a sort of hood. This 

 invites comparison with the genera mentioned above, but for the fact that 

 the opposition of stamens and sepals would be normal, according to the rule 

 of alternation, if any true petals were present. In Banksia (Fig. 1 137), there 



