TEE STUDY OF THE EVOLUTION OF THE ELOWEK. 299 



The line of anatropal advance and specialization is accompanied by the transition 

 from the bi- to the uni-tegumentary condition. 



Benson *, in 1894, found all stages from two completely free integuments in Eagaceie, 

 to two more or less fused in Gasuarinacese and Corylacese. Pechoutref, in 1902, 

 demonstrated a similar series in Kosacege and both the uni- and bi-tegumentary con- 

 dition in one genus, SpircBa. 



A third series occurs in the Eanunculacese, whilst one or two integuments characterizes 

 the Dilleniace^e and Ochnaceae. A single integument is found, for example, in the 

 Triuridales, Ceratophyllacese, Empetracese, Aquifoliacese, Cochlospermacese, Loasaceae, 

 TJmbelliflorEe, and prevails almost exclusively in nearly all the cohorts of the 



Sympetalse. 



A third line of advance lies in the direction of the reduction in bulk of the nuccllus. 

 The axile rows are numerous in the Eosacese, few in the Araliacete, and reach their 

 minimum development in Lilium, Hehcingia, and Imila. 



Ovular Connections. — Where each loculus contains several series of ovules, as in 

 the Saxifragacese, the main placental bundles branch repeatedly throughout their course 

 on their outer side ; branches of the second and third order arise from these and the 

 ultimate branchlets supply the ovules— one to each. Transitional vascular stages, 

 accompanying the change from the multiseriate to the biseriate arrangement, have not 

 been studied ; but when the ovules are in two series, as in Leycesteria, the system is 

 reduced, each main bivalent axial bundle produces short branches on either side, 

 supplying ovules in the right- and left-hand loculus respectively— one branch to each 

 ovule. "When the ovules in the double series are reduced to a terminal pair, rendering 

 each loculus biovulate, the branches are also reduced to two as in Aralia and the 

 Hamamelidaceai. The further change from the biovulate to the uniovulate loculus 

 involves several instances. Thus the change from Aralia or Corylopsis, where one of a 

 pair of ovules is sterile, to Samamelis, where one placenta of each pair is barren, is 

 accompanied by the abortion of the branch pertaining to the barren placenta and the 

 ovule receives a single bundle. But where the single ovule is axially borne, the ovular 

 trace is frequently double, receiving a vascular supply equivalent to that of a pair of 

 axially borne ovules. In Davidia, the branches uniting to form the ovular trace arise 

 from several small bundles extending in the right- and left-hand septa and pertain to the 

 right- and left-hand sides of the carpel; in Cornus, the branches arise from bivalent 

 peripheral bundles ; and in Helwingia, where the median branch from an axial bundle 

 forms the ovular trace, the axial bundle represents two united mai-mal bundles of tlie 

 same carpel, so that the median branch may be considered bivalent. 



In more advanced stages of reduction, where the septa disappear and the ovary 

 becomes uniovulate, various anomalies occur, such as have been described m detail for 



Viburnum, Corokia, Griselinia, and Nyssa. 

 It is highly probable that a parallel series of stages will be found leadmg towards 



* M. Benson in Trans. Linn. Soc, Bot. 2ncl ser. iii. (1894) 409-424, t. 67-7:i. 

 t r. Pechoutre in Ann. Sci. Nat., Bot. 8°^« se'r. xvi. (1002) 88. 



