1 84 METABOLISM 



formation of the enzyme is often, though by no means always (Went, 1901), 

 regulated by the need for it ; when the product of the activity of the enzyme, 

 or perhaps only a body like it, is present in the plant, the enzyme is not 

 required. Doubtless the formation and dissolution of starch, for example, is 

 regulated in higher plants in this way, and similar self-regulating processes 

 are likely to occur, other than those associated with metabolism. 



Without entering further into a discussion of the other enzymes (on which 

 an extensive modern literature exists, e.g. Fermi and Buscaglioni, 1899, San- 



GUINETTI, 1897, MALFITANO, I9OO, KOHNSTAMM, I9OI, BRUNSTEIN, I9OI), We 



can easily see how greatly the possession of such a secretion capable of bringing 

 about extra-cellular solution facilitates the distribution of Fungi in nature, 

 where suitable nutritive solutions are not always to be obtained ready to hand. 

 They possess not only dissolving but also splitting enzymes as well. 

 Disaccharides must be spht up before they can be used, for if an organism 

 such as Bacillus perlibratus (Beijerinck, 1893) possessed no sugar-splitting 

 enzymes it could thrive only on dextrose and levulose, but would be debarred 

 from using maltose, cane sugar and lactose. 



The heterotrophic plants we have hitherto been considering are sapro- 

 phytes, i. e. they exist in nature on the dead remains or the excreted meta- 

 bolic products of the animal and plant worlds. In a certain sense we may 

 describe the biological group of plants known as carnivors as an intermediate 

 class between the saprophytes which we have been studying and the parasites 

 yet to be discussed. 



These much investigated and well-known carnivorous, or, as one may 

 more specifically term them, insectivorous plants have excited the greatest 

 interest not merely among botanists but the general public as well, on account 

 of the contrivances by which they obtain possession of their nutriment and by 

 the methods they use in digesting it. But from a purely physiological point 

 of view they are so gradually transitional to other types that they might be 

 classed equally correctly under saprophytes or under autotrophic plants. We 

 cannot enter here into a description of the morphological characters of these 

 plants ; we may refer to Figs. 33-35 and to the special treatises on the subject, 

 especially those of Goebel (1891-3), and only remark that in order to catch 

 small insects essentially three types of apparatus are employed, viz. (i) pitchers, 

 or trap-like cavities, such as the pitchers of Nepenthes (Fig. 33), Sarracenia and 

 Cephalotus, or the bladders of Utricularia (Fig. 35) ; (2) closing traps, that is, 

 organs which catch insects by active movements, Dionaea (Fig. 34) and Aldro- 

 vanda ; (3) sticky hairs [Drosera, Fig. 156, Lecture XXXVIII), Drosophyllum, 

 Pinguicula. Combinations of these contrivances also occur. 



In certain simple cases, such as the bladders of Utricularia, the animals 

 remain for a long time alive, die at last of hunger and decompose in 

 situ as the result of the action of Bacteria. We may assume that their 

 excrement at first and, later, the products of the decomposition of their 

 bodies are used by the plant as sources of nitrogen, so that a truly car- 

 nivorous habit does not here exist. A similar state of affairs appears to 

 hold good for Sarracenia and Cephalotus, and hence we may group these 

 plants in the same category with others which possess water reservoirs, 

 especially the Bromeliaceae already mentioned and Dischidia rafflesiana, 

 with its remarkable water pitchers ; in all these cases animals are invariably 

 present in the water reservoirs, their products of decomposition being in course 

 of time absorbed by the plant. The typical carnivorous plants are distinguished, 

 however, by the fact that they secrete proteolytic enzymes, for the most part 

 along with acids, by means of which they are able to dissolve proteid materials. 

 Further differences exist between the different types, inasmuch as in some 

 species the protease and the acid are always being secreted, whilst in other cases 

 one or both substances are produced only in response to a stimulus, especially 



