i86 METABOLISM 



The question whether carnivorous plants are benefited by being fed on 

 insects has often been discussed. It is a well-established fact that they can 

 not only exist but also thrive without any such nutriment. On the other 

 hand, it has often been shown that such food materials produce a favourable 

 result when given in sufficient amount. In the case of Busgen's (1888) re- 

 searches, for example, the increase in growth of fed Utricularia shoots was double 

 that of the unfed ; and the same author (1883) showed that Drosera exhibited 

 remarkable results after being nourished on meat from the seedling stage right 

 up to the formation of seed. The dry weight of the plants which had been fed 

 was one and a half to three times that of the unfed ; the fed plants had three 

 times as many inflorescences, and formed five times as many capsules. There 

 can be no doubt that carnivorous plants are autotrophic so far as the acquisition 

 of carbon is concerned ; all of them have abundant chlorophyll and can thrive 

 without receiving any proteid food. The favourable action of the latter cannot 

 well depend on the gain in organically combined carbon ; it is more probable 

 that it originates from the nitrogen supplied or certain nutritive salts. It is 

 possible that feeding with insects is beneficial simply because a larger amount of 

 combined nitrogen and materials of the ash is introduced in this way into the 

 plant than could be obtained from the soil only. It is even more probable, 

 however, that the quality of the materials which are absorbed by the leaves 

 is of significance ; for instance, the obtaining of organically combined phos- 

 phorus or organic nitrogen may be the end specially aimed at. In the latter 

 case, which, without more exact proof may be regarded as the point of chief 

 importance, we must regard peptones as especially valuable. It is, in fact, 

 perfectly obvious that carnivorous plants are peptone-feeders, or in other words 

 that they are better nourished when the nitrogen is in the form of peptone 

 than in that of nitrate or ammonia. This idea is well worthy of investigation ; 

 meanwhile, we must depend on analogies and it is important to remember that 

 there are peptone organisms not only among Fungi, which are also dependent 

 on carbon in the organic form, but also among green autotrophic plants. 

 Beijerinck (1890) and Artari (1899) have shown that certain Lichen-Algae 

 (compare Lecture XIX) prefer peptone to any other compound of nitrogen. 



The biological position of carnivorous plants is at present somewhat in- 

 definite, still it is fairly well settled that their proper place is among what 

 may be termed ' nitrogenously heterotrophic ' plants. As already remarked, 

 we have another series of heterotrophic plants to consider, namely parasites. 

 These forms are united with saprophytes by many transitions. Fungi are known 

 (compare De Bary, 1884) which live as a rule saprophytically, but which still 

 have the power of penetrating living organisms and of taking their nourishment 

 from them. As examples of such ' facultative parasites ' may be taken Peni- 

 cillium glaucum and other Mould-fungi which enter into wounds in ripe fruit and 

 cause putrefaction in them ; further, Sclerotinia sclerotiorum may be mentioned, 

 as a fungus which can carry out its entire cycle of development as a saprophyte, 

 and does so in nature not infrequently, but which, after attaining sufficient 

 vigour, can live also as a parasite on many plants. The converse condition 

 is also known, viz. Fungi, which usually live as parasites, but which can 

 nourish themselves saprophytically (' facultative saprophytes,' e. g. Phytoph- 

 thora omnivora and many Bacteria), and research has certainly shown that 

 very many, if not all, parasites can grow and increase without the aid of 

 their usual hosts. The probability of such a result is very varied in different 

 degrees of parasitism. 



The lower stages of parasitism include such Fungi as appear to attack 

 many different species of plants, as, for example, the already mentioned Phy- 

 tophthora omnivora, which is parasitic on Fagus, Sempervivum, Oenothera and 

 other plants, and Sclerotinia sclerotiorum, which is apparently able to attack 

 all succulent parts of plants. These parasites are distinguished from ordinary 



