822 REPORT — 1901. 



from those in which these extinct plants throve. The factors which determine the 

 success or otherwise of an organism or group of organisms at any period must 

 always be complex, and no exception can be claimed for plants in their struggle 

 for mastery. But looking at the succession of ])lant-l ile in the world in relation 

 to the known diminution of water-surface and increase of land-area, and the 

 consequent differentiation of climates, we cannot but be convinced that of these 

 factors water is one which has had supreme inlluence upon the evolution of the 

 faeies of the plant-life that we see to-day. I think the statement is warranted 

 that the Angiosperms have become dominant in great measure because in their 

 construction the problem of the plant's relationship to water on a land-area has 

 been solved more satisfactorily than in the case of the groups that preceded 

 them. 



The seed-character — and the flower which it involves — distinguishes the 

 Angiosperms. AVhat, then, are the relationships to water which the formation of 

 seed implies and through which the Angiosperm has advantage ? 



Two prominent risks in its relation to water attach to the process of sexual 

 reproduction in a plant of the type of heterosporous Pteridophytes. Firstly, that 

 of failure of moisture on the soil sulEcient to promote germination of the spores ; 

 secondly, that of failure of moisture on the soil sufficient for the passage of the 

 spermatozoid to the ovum. In addition there is the risk of failure of the fall of 

 microspores and megaspores together upon the soil. In the Angiosperms such 

 risks are practically abolished in the formation of flower. The stigmatic surface 

 of the style itself provides a secretion — the more copious in a diy and sunny 

 atmosphere — to moisten the pollen-grain and stimulate germination, and for the 

 spontaneous movement of the spermatozoid is substituted the passive carriage of 

 the male gamete to the ovum by the agency of the pollen-tube. Possible failure 

 of pollination is, too, provided against hj the complex mechanism of the flower in 

 the highest forms in relation to insect-visits. The sexual act, then, might, wc 

 conceive, gradually become more and more difficult of consummation to the 

 Pteridophyte as the area of dry land increased. To the seed-plant it was more 

 secure by its independence of the presence of free water. The failure of perform- 

 ance of the function of sexual reproduction may have hastened the disappearance 

 of Pteridophytes before the advance of the Angiosperms. 



But if this flower-mechanism relieves the Angiosperm from rislis in the per- 

 formance of the sexual act, it imposes a new duty upon the plant, that of nursing 

 the embryo within the sporangium. This involves a water-supply of a kind not 

 demanded in the Pteridophytes, and we may gain some idea of the importance of 

 this by a comparison of the trivial vascular system required to supply through the 

 stamen the pollen-grain, with the copious system thnt traverses the gynaeceum for 

 the ovules. It is, however, to the ovule — the immediate nursery of the embryo — 

 that we must look for special indications of this water-relationship of which I 



speak. 



Perhaps no organ has given rise to more discussion than this characteristic one 

 of flowering plants. To most of us I believe the controversy over its axial or foliar 

 nature will be, in a measure, historical only. All recent investigations of sporangia — 

 and to no one does Botany owe more in this respect than to Bower — tend to confirm 

 the view that it is, and always has been, an organ sui generis. To that category 

 the nucellus of the ovule is now pretty generally admitted. It is the body of a 

 sporangium. But the nature of the tegumentary system and of the funicle which 

 give the ovule so distinctive a character is still the subject of disagreement.* 



I do not share a view which sees in the integuments or other parts of the ovule 

 anything of an axial or of a foliar nature. To me the funicle is a sporangiophore — 



' Scott's discovery of a bracteal investment to the megasporangium in Lepido- 

 carpon is an interesting one in relation to the question of the enclosm-e of sporangia. 

 It shows how in the Lepidodendrese a coveruig of the sporangium could be developed, 

 much in the same way as a carpellary envelope in Angiosperms. Whether the 

 ovular integument or the ovarian covering in Angiosperms was the earlier develop- 

 raent is open to discussion. I am disposed to give precedence to the ovular coat. 



