Chapter X 

 DROSERA 



Number of species. — Geographical distribution. — Habitat. — Form and habit of the 

 plant. — Unfolding movements of the leaf. — The leaf (Form. Anatomy. Appendages. 

 Tentacles. Sessile glands, origin and structure, function. Locus of absorption. Other 

 glands). — Reproduction. — Carnivory, early observations. — Mucilage, origin. — Move- 

 ments of the tentacles (Early observations. Nitschke and Darwin). — Direction of bend- 

 ing. — Duration of response. — Leaf blade not receptive to stimulus. — Path of stimulus. 



— Intensity of stimulus. — Mechanism of movement. — Behre's studies. — Aggregation. 



— Digestion. — Enzymes. — The significance of carnivory for the plant. 



The genus Drosera contains more than 90 species found in almost 

 all parts of the world. It reaches its greatest development in Australia 

 and is well represented in S. Africa. The most widely known, at least 

 historically, is the common sundew, ros solis, D. rotundifolia, the plant 

 which chiefly formed the subject of Darwin's extensive studies. This 

 and its allies, D. anglica, D. intermedia and filiformis, also well known 

 in the North Temperate zone, are modest representatives of the genus 

 as compared with such forms as D. gigantea of Australia or D. regia of 

 S. Africa. 



Habitat. — It is very generally understood that Drosera grows 

 where the soil is poor in nutrient substances. Such a statement ap- 

 plies fully enough to the best known species of the northern hem- 

 isphere, D. rotundifolia, intermedia, filiformis, etc., but seems not to be 

 true of some species such as D. Whittakeri of S. Australia, where I 

 saw it growing on wooded slopes with a general vegetation. Even this, 

 however, though probably a richer soil than that of a sphagnum 

 swamp, is not a rich soil. One commonly linds D. rotundifolia in any 

 swamp where Sphagnum grows, and it grows plentifully in the chinks 

 of partially decayed floating or stranded logs, a favorite place. In the 

 Sequoia National Park, California, it is found in the wet open mead- 

 ows surrounded by Sequoia gigantea, growing on a dense floor of moss 

 (not Sphagnum). A more accurate picture is afforded by Weber (1902) 

 in his monograph describing the great swamps of Augustumal, in the 

 delta of the Memel River. There is in this swamp, as of course in 

 swamps elsewhere, a zonation of the vegetation. As one proceeds from 

 the margin to the middle, one finds that the ash content of the soil 

 and soil water becomes more and more reduced. It is only in the more 

 central parts that D. anglica and D. rotundifolia are to be found, and 

 these are the parts which are most lacking in salts. The vegetation 

 here consists of Spagnum with Cladonia uncinalis, Scirpus caespitosus, 

 Eriophorum vaginatum, Scheuchzeria palustris, Rhynchospora alba, 

 Vaccinium oxy coccus, and Andromeda polifolia — and therefore of few 

 species. This habitat was found to have a soil with the following com- 

 position in absolute and relative terms. In quoting these data, Schmld 



Augustumal Diluvial clay 



