176 THE FERTILISATION OF FLOWERS. [PART III. 
no means fully explained yet. I hope at least to advance our know- 
ledge of the means by which the elastic tension of the parts is 
produced and controlled. Medicago sativa has, like Sarothamnus 
scoparius and Genista tinctoria, explosive flowers. As in these 
latter, the staminal column before explosion is inclosed within 
the carina, whose upper edges cohere together; and when an 
insect presses down the carina the column not only emerges but 
springs violently up and is unable to return to its former place. 
As in G. tinctoria, the reproductive organs after explosion are 
pressed against the vexillum and are not affected by further insect 
visits. But Medicago sativa differs from these two plants by having 
both the elastic tension and the force which restrains it localised in 
different parts of the flower. 
Fic. 56.—Medicago sativa, L. 
1.—Young (virgin) flower from below. 
2.—Ditto, from above, after removal of the vexillum and the upper half of the calyx. | 
3.—Inner view of right ala. 
4.—Carina seen obliquely from above. 
5.—Exploded flower seen obliquely from above ; the vexillum and upper half of the calyx ha 
been removed (x 84). 
a, calyx; b, vexillum ; ¢, ala; d, its claw; e, invaginated process of ala, directed inwards and 
forwards ; f, its externa] opening; g, digitate process or lobe of ala, directed inwards and back- — 
wards ; h, carina ; i, invaginated pouches in the carina, which receive the alar processes, (e); &, the 
coherent filaments; 1, the upper free stamen ; m, entrances to the honey; , anthers; 0, stigma. 
While in Sarothamnus it is the style only which acts as a 
spring, in Genista tinctoria the column tends to spring upwards, and 
the ale and carina together tend to spring downwards. In Medicago 
sativa the elastic tension resides almost exclusively in the upper 
stamens. ‘his can be shown clearly by dividing the upper from — 
the lower stamens, whereupon the upper part of the bundle curves 
still more strongly upwards, and the lower part sinks down till it 
stands almost horizontal. 
In Medicago sativa it is not the coherent upper margins of the 
