TRANSACTIONS OF SECTION K. 857 



(B) 1. The union of two ancestral cotyledons into a single member may have 

 arisen from the adaptation of the earlier monocotyledons to a geophilous habit. 

 This view is supported by : 



(i) Comparison with the dicotylous species in which the cotyledons 

 are united for a considerable distance from the base upwards. VV ith 

 the single exception of the mangrove, these are all highly specialised 

 geophytes. They all have a subterranean and greatly shortened vertical 

 axis, which is often tuberous. 



(ii) The fact that all dicotyledons with a single seed-leaf are well- 

 marked geophytes. 



(iii) The probability that the union of cotyledons is of service to 

 the geophyte by reducing the expenditure of material in assimilating 

 surfaces during the first year of growth. Such reduction is forced 

 upon plants of this habit by the short season of growth in their native 

 climate, and the prime necessity of forming underground organs before 

 the beginning of the dead season. 



( B) 2. Many features characteristic of mature monocotyledons can be explained 

 as distinct adaptations to a geophilous habit, or as the necessary consequence of 

 such adaptation. 



(i) The linear leaves of bulbs, with their parallel venation and 

 broad bases, are peculiarly adapted to rapid elongation and penetration 

 of the soil. 



(ii) The stem of monocotyledons shows many concentric circles 

 of leaf-trace bundles. These are the almost inevitable consequence of 

 the insertion on a squat axis of closely packed leaves with broad bases. 

 From each leaf a number of parallel traces enter the axis in the 

 segment of a circle. 



(iii) The substitution of short-lived roots in immediate connection 

 with the leaves, for a single branched tap-root is characteristic of low- 

 growing plants exposed to alternating periods of activity and repose. 



(iv) The formation of albuminous seeds, so general among mono- 

 cotyledons, seems very commonly correlated with the geophilous habit 

 in Dicotyledons. In geophilous species the embryo is usually small 

 and little developed, probably because it has no time to grow larger in 

 the short growing season. Such seeds usually require a long period of 

 maturation before they germinate. 



ii. A Consideration of the Bearing of Fertilisation Phenomena and 

 Embryo Sac Sti-ucture on the Origin of Monocotyledons. By Ethel 

 N. Thomas. 



There exists great uniformity among Angiosperms in these respects. 



The development of the embryo sac shows it to be a megaspore. 



Comparison with the germination of other megaspores shows that a great gulf 

 exists between Angiosperms and all other groups of the vegetable kingdom. No 

 distinction is found in this respect between monocotyledons and dicotyledons. 



A few exceptional cases of embryo sac structure are known. 



(a) Those in which normal productions arise from less than the usual number 

 of constituents. Do these indicate what is essential to the process ? 



(b) Those in which normal productions arise from more than the usual com- 

 ponents. Are these primitive ? 



The origin and modes of production of the Angiospermous endosperm are of 



