CLASSIFICATION OF PLANTS BY EVOLUTION. 



135 



as explained in my previous lecture ; as another instance Atragene 

 alpina (allied to Clematis) may be added. In this flower a groove on the 

 filament secretes honey, and a transition from stamens to petals can be 

 easily traced, all being enveloped in the four large purple sepals. In 

 these respects Atragene shows an advance upon Clematis. What brought 

 it about ? The theory the present writer would propose is that the 

 developing of the honey-secreting surface on the filaments, and their 

 alteration to petals, are the direct consequence of the irritations set up by 

 the bees which visit this flower for pollen and honey. 



Protoplasm is a highly ' irritable " substance and " responds " in many 

 ways to external influences, including mechanical weights, pressures, 

 tensions, &c, and the theory is based on innumerable coincidences 

 coupled with much experimental evidence.* Thus honey-secreting 

 organs are always precisely where the proboscis or tongue of the insect 

 can most easily reach them ; and as in accordance with evolution nothing 

 is ever made in anticipation or for direct use of any other being, we 

 know of no other cause than the actual probing of the insect itself. Thus, 

 we find the sepals producing honey in the Lime, the petals in Buttercups, 

 Columbine, Larkspur, and Aconite, the stamens in Atragene, the carpels 

 in Caltha, &c. But in the great majority of instances, as the proboscis 

 goes down to the receptacle, it is this which produces glands, as, e.g., two 

 in Wallflower, five in Geranium, an entire disk in Maple &c. 



The formation of honey-secreting glands &c. and the conversion of 

 stamens into petals are, then, the first supposed results of insect agency. 



The next to be considered is the "cohesion " which often takes place 

 between the parts of a whorl. It is particularly conspicuous in the 

 corolla; we thus get "tubes" and "bells," as in the Primrose and 

 Campanula &c. It is believed that the freedom of parts always pre- 

 ceded such unions ; as leaves on a shoot are always free. 



2. Primitive flowers, such as the Buttercup, have often numerous 

 stamens and carpels spirally arranged, after the manner of leaves on a 

 shoot. These, as is well known, are arranged in " cycles " or groups of 

 2, 3, 5, 8, &c, and a long spiral of many stamens or carpels of course means 

 several " cycles " of such parts ; and the reduction to one or two cycles is 

 one of the commonest features in flowers. In Dicotyledons the whorls 

 are usually in fives or fours, and in Monocotyledons in threes. 



The number of carpels is usually further reduced. Thus, while a 

 Geranium has 5 sepals, 5 petals, 5 + 5 stamens (i.e. in two whorls), and 

 5 carpels, a Potato blossom has only 5 stamens and 2 carpels. 



The reduction of carpels is common with high specialisation. Thus 

 in the two "irregular" flowers of Aconite and Larkspur the carpels are 

 reduced to 3, 2, or even 1 ; yet both these belong to Banunculacece. 



3. The floral receptacle is usually only a rather enlarged summit to 

 the pedicel, or flower- stalk, as in a Buttercup. It becomes very large in 

 a Strawberry in order to carry the numerous fruits (achenes). 



In addition to this terminal expansion it may extend laterally round 

 the pistil in the centre. This is called the " receptacular tube," and 

 assumes various forms. Its primary function is to secrete honey. This 



* The reader is referred to The Origin of Floral Structures and TJie Making 

 of Flowers, by the present writer. 



