128 VARIATIONS IX NUMBER OF SPORAXGIA 



initiated frequently do not come to full maturity this further fact is to be 

 borne in mind : that it is common in the ascending series of plants to 

 find a larger number of primordia initiated than the individual can bring 

 to full development. Consequently it will be necessary to discriminate 

 between those imperfect parts which represent such as were fully matured 

 in the ancestry, and those supernumerary primordia which do not represent 

 parts ever actually matured in previous generations. In point of fact, 

 unperfected primordia may, in some cases, really represent an exuberant 

 tendency to progress, and cannot necessarily be assumed to indicate a 

 state of reduction. 



(K) The reduction or arrest of apical or intercalary growth in parts 

 bearing sporangia has doubtless been a potent influence in the past, but 

 it is somewhat difficult to point to concrete examples of it. The general 

 fact that the floral axis in Phanerogams is abbreviated, while the axis of 

 the strobilus in Pteridophytes is frequently endowed with continued apical 

 growth, points to the probability of arrest of that apical growth. An 

 example of it within a near circle of affinity is seen in the genus Lycopodium : 

 for in the section Selago the apical growth of the axis is unlimited, and 

 the number of possible sporangia borne on a given axis is unlimited also : 

 in other sections of the genus which are held to be more specialised, as 

 the strobilus becomes more strictly differentiated from the vegetative region 

 the apical growth is arrested early, and the number of sporangia produced 

 upon it is limited also. In floral details this is illustrated in some families 

 of Angiosperms : thus the Ranunculaceae include such forms as Myosurus, 

 with its elongated receptacle and indefinite number of carpels, and Actaea, 

 with its abbreviated axis and only a single carpel. It seems probable that 

 here also arrest of the apical growth has been associated with reduction 

 of the number of sporophylls and of sporangia. 



(/) Fusion of parts which bear sporangia is not so susceptible of illus- 

 tration in the Pteridophytes as in the Angiosperms. A most convincing 

 series of reduction is laid out in the Araceae by Engler, involving fusion 

 of stamens, accompanied by reduction in number of the pollen-sacs (Nat. 

 Pflanzenfam. II. i., p. 107). Many examples might also be quoted from 

 other Angiosperms, e.g. Cucurbitaceae, Euphorbiaceae, where fusion of parts 

 is connected with reduction in number of the sporangia which they 

 bear. A suppression of branchings of sporangium-bearing parts may also 

 be a factor : it has at times been assumed in theoretical writing, but it is 

 difficult to give conclusive examples of it. 



(j) Indirectly the arrest of apical growth and flic partial or complete 

 suppression of branchings in the non-sporangial region may affect the 

 number of sporangia produced. The abortive buds at the base of 

 inflorescences of Aconitum, for instance, or the abortive spikelets in 

 Cynositrns are examples of potentially larger numbers of spore-producing 

 branches arrested before they bear sporangia. Such arrests of growth 

 and of branching may have been more prevalent sources of change than 



