Intro- —3— duction 



through agencies such as mycorrhizal fungi. Our so-called carnivorous 

 plants are therefore not peculiar in this habit. 



What then distinguishes the carnivorous plants from the rest of the 

 plant world? Why should we still share the feelings of the naturalists 

 of the 1 8th century who regarded them as miracula naturae? We do so, 

 I think, because a carnivorous plant in the sense here meant is one 

 possessing a trap which, though merely a constellation of structures and 

 functions, many of them conmion enough elsewhere among plants, is a 

 special organ for the capture and digestion of animal prey, thus turning 

 the tables on animals, which directly or indirectly are herbivorous. 



But may these traps as such be regarded as something unique? The 

 answer to this question must be sought in such analogs as we may find 

 among plants in general. 



Pitfalls in the form of pitchers are of rather widespread occurrence. 

 In some flowers the corolla is tubular and the inner surface is supplied 

 with downward pointing hairs, and there is emitted a luring, if not al- 

 luring, smell. Flies are attracted and caught, but after effecting polli- 

 nation, and the hairs having withered, are released (Aristolochia). In 

 other flowers one or more members of the perianth are tubular and se- 

 crete and hold nectar {Aquilegia, Marcgravia, Delphinium). Perhaps 

 the closest parallel is found in the pitcher leaves of some species of 

 Dischidia, a tropical genus of the old world. They are invaded by 

 adventitious roots from nearby stems of their own plant, and are often 

 occupied by ants who use them as shelters. Probably in their native 

 habitats they often contain moisture available for their invading roots. 

 An inturned marginal rim surrounding the narrow mouth reminds one 

 of the rim of the carnivorous pitcher, but it seems to have no well 

 marked special function. In some species of Dischidia the pitchers are 

 represented merely by dished leaves facing each other. At the other 

 extreme Dischidia pectinoides has a double pitcher, one inside the other, 

 according to Goebel. Lathraea squamaria, a root parasite of Europe, 

 has hollow leaves, the hollow lined with glands. Goebel regards them 

 as reservoirs for reserve stuffs. The upper leaf lobes of Azolla are also 

 hollow, but these are inhabited by Anabaena azollae in symbiotic re- 

 lationship. Among the liverworts are species in which the leaves are 

 partly converted into "water sacs" (Goebel), notably Frullania 

 cornigera of New Zealand, though our own species offer sufficiently 

 good examples. Lejeunea behaves similarly, but the sacs are simpler. 

 Most impressive are Colura and Physiotum. In P. majus^ occur nearly 

 closed sacs the mouths of which are guarded by two lips closed to- 

 gether like the lips of a mussel shell (Goebel). Moreover one of the lips 

 is moveable, being provided with a hinge region, thus serving as a valve. 

 Precisely how this valve works is not clear. Goebel, to whose account 

 I am indebted, points out that such an arrangement is known only in 

 Utricularia, but it must be remembered that this comparison loses some of 

 its cogency for the reason that Goebel thought the valve of the Utric- 

 ularia trap to be a simple check valve. There is no evidence that these 

 arrangements in the liverworts indicate a carnivorous habit, though 

 they are inhabited, like any liverwort or moss, by protozoa, nematodes, 

 etc. That they are water holders is evident. 



The common teasel {Dipsacus sylvestris) has been regarded in all 



