EVOLUTION OF PLANTS, AND THE DIRECTIVITY OF LIFE. 553 



THE EVOLUTION OF PLANTS, AND THE DIRECTIVITY OF 

 LIFE, AS SHOWN BY THE REPRODUCTIVE ORGANS. 



By Rev. Professor Geo. Henslow, M.A., F.L.S., V.M.H. 



[Read October 21, 191 3 ; Mr. Bernard W. Crisp in the Chair.] 



In my last lecture I dealt with the evolution of the vegetative organs 

 of plants. I propose now to consider, theoretically, what may have 

 probably been somewhat of the process by which the floral organs 

 have been evolved. 



The belief is universally accepted that the sepals, petals, stamens, 

 and carpels of flowers are fundamentally identical with leaves and 

 are interchangeable with them ; as may be witnessed by the Green 

 Rose, wherein every organ is represented by a green leaf. 



If we search among the relics of plant-life in the strata of the 

 earth, we soon find that the geological history of plants is so imperfect 

 that not very much has been discovered to throw light upon the 

 origin of flowers. Many reasons exist, however, for believing that 

 the conifers and other members of the " naked-seeding " plants, called 

 therefore " Gymnosperms," have descended from the higher seedless 

 or spore-bearing Cryptogams, such as Ferns or fern-like plants. 



The Gymnosperms, we have every reason to believe, gave rise 

 to Dicotyledons ; but the links are unknown between these naked- 

 seeded plants, having no carpels to include the ovules and carpellate 

 plants. The Monocotyledons are undoubtedly a later evolution, 

 having been derived from aquatic Dicotyledons. 



The first change, from leaves into parts of flowers, is to reduce 

 them to scale-like structures, as in our Pines, which have two kinds 

 of foliage, needle-like and scale, leaves. There are no sepals or 

 petals ; the scale developing anther-cells with pollen is wholly green 

 in the Cypress, with four yellow anther-cells ; but in the Pine the 

 whole scale is yellow, so this colour seems to have been the most 

 primitive change from green. 



Ovules arise as outgrowths from the " carpellary scale," a branch 

 from the vascular bundle being clothed with tissue and converted 

 into the female structure. A reverse may occur in foliaceous flowers ; 

 for in a " monstrous " Mignonette the carpellary leaf bore little 

 cup-like extensions from the vascular cords. Even cabbage-leaves 

 will sometimes produce similar excrescences ; these also show the 

 homology with ovules. 



When, however, we search for previous causes, and ask how and 

 why the internal tissue of a leaf -blade can become pollen-grains, 



