8o INTRODUCTION 



themum leucanthemum was visited by a small slug (Limax laevis Miill.) which in wet 

 weather discharged the task of pollination. In one small area Ludwig found this 

 slug on hundreds of capitula, and it seemed as if the white ray-florets formed the 

 attraction, as they were very greedily devoured by the visitors. Ludwig thus com- 

 ments on this observation : ' It proves that plants which lack the customary agents 

 of pollination, when continuous rain occurs during the flowering season, and would 

 otherwise produce no fruit, may find in slugs effective substitutes for insects, which 

 are only active in dry weather.' Other botanists have repeatedly observed that snails 

 and slugs visit and pollinate flowers, e.g. : Engler (' Monogr. Phanerog. auct. A. et 

 Cas. de Candolle,' V, 2, p. 30) substantiates this for Anthurium coriaceum and 

 A. martianum, as he observed small slugs visiting them in the aquarium of the Botanic 

 Garden in Munich. Trelease (Amer. Nat., Boston, xiii, 1879), in North America, 

 saw small snails carrying pollen about on Symplocarpus foetidus Salisb. 



{d) Plants with Insect-pollinated Flowers. Entomophilae (En). 



In his 'Das entdeckte Geh.' (pp. 9-21) Sprengel has set forth the essential 

 characters of insect-pollinated plants (cf. p. 5). An exhaustive account is therefore 

 superfluous, and having regard to the present state of our knowledge, it will only 

 be necessary to emphasize the most important points. 



In contrast to the dusty pollen of wind-pollinated plants, for which the name 

 ' flower dust ' is very appropriate, insect-pollinated flowers possess adhesive pollen. 

 Its outer coat is beset with small spines, warts, pits, grooves, needle- or hair-Hke 

 structures, in short, with numerous small processes by which its adhesion to the 

 bodies of visitors is specially favoured. At times the pollen-cells are bound together 

 by threads of a delicate sticky substance known as Viscin, by which adhesion is 

 rendered still easier. Such threads of viscin occur, e.g. on the pollen-grains of 

 Oenothera, Epilobium, and other Onagraceae, among species of Rhododendron, &c. 



The size of pollen-grains varies greatly. It is mostly given in micromillimetres 

 (i /i=o-ooi mm.). Thus, according to Kerner ('Nat. Hist. PI.,' Eng. Ed. i, II, 

 p. 97), the size in Myosotis alpestris is 0-002 5-0-0034 mm., in Mirabilis Jalapa 

 0-22-0-25 mm., so that in the latter plant it is a hundredfold that of the former. The 

 average size is about 25-100 fi. 



The pollen of all flowering plants, hydrophilous forms alone excepted, is at once 

 damaged by water. Kerner has paid special attention to the many ways in which 

 pollen is protected. I therefore enumerate the protective arrangements as distin- 

 guished by him ('Nat. Hist. PI.,' Eng. Ed. i, II, pp. 104-29). 



1. The anthers are covered by a protective roof. This is eff"ected in 

 one of the following ways : 



{a) Campanulate, urceolate, basin-shaped, or cup-shaped flowers, depending from 

 curved stalks, e.g. species of Calluna, Vaccinium, Campanula, Pulmonaria, and Con- 

 vallaria ; Atropa Belladonna ; species of Galanthus, Leucojum, and Fritillaria. 



{b) Curvature of the main floral axis; the flowers are thus inverted as before, 

 so that the stamens are sheltered by the petals : Berberis, Prunus Padus. 



(c) TTie flowers or inflorescetices betid over periodically, their stalks (or the elon- 

 gated inferior ovaries) curving downwards at night and in bad weather, so that once 



