Chapter VIII — 101 — DrosophyUum 



The third layer (limiting layer of Penzig) caps a mass of short 

 irregular tracheids constituting the expanded end of a strand of vas- 

 cular tissue extending through the stalk and communicating with the 

 vascular tissues of the leaf. This strand consists of both xylem and 

 phloem elements (Fenner contra Meyer and Dew-eyre) affording, 

 according to Fenner, not only a pathway for water but, in the case of 

 the phloem, for the transmission of stimuli to the neighboring sessile 

 glands, which have been shown to show secretory activity in response 

 to such stimulus received from the stalked glands. The stalk itself is 

 made up of the epidermis and an underlying course of parenchyma, sur- 

 rounding the vascular strand. The capping secreting cells contain 

 brilHant red coloring matter, interpreted as an optical lure for insects, 

 and when the capital bears its shining droplet of clear mucilage, which 

 acts as a Hght collecting lens, the glands appear as brilUiant red dots. 

 The sessile glands have no such coloring matter. These {14 —6) have 

 the same structure as the stalked glands, differing only in the absence 

 of the stalk. Occasionally an intergrading condition is met with; 

 Goebel found one such, with a very short stalk. The sessile glands 

 are usually oval, generally smaller, and have a less extensive contact 

 with the vascular system. Each gland, however, is underlaid by a 

 group of cavernous looking tracheidal cells, with no protoplasmic con- 

 tent, evidently an important part of the gland but with what function 

 we do not know. If Fenner saw this feature, he regarded it as the 

 end of the tracheidal system. For there is also to be found at the base 

 of each gland the end of a branch of the vascular system. These 

 glands are devoid of a mucilaginous secretion, as of coloring pigment 

 and even of chlorophyll, for they appear whitish. 



The mucilage secreted by the stalked glands is peculiar, in that it 

 is not readily drawn out into slender viscous threads, but is easily 

 pulled off the gland by a touch of even a needle point as Darwin ob- 

 served. "From this peculiarity, when a small insect alights on a leaf 

 of DrosophyUum, the drops adhere to its wings, feet or body, and are 

 drawn from the gland; the insect then crawls onward and other drops 

 adhere to it; so that at last, bathed by the viscid secretion it sinks 

 down and dies, resting on the small sessile glands with which the sur- 

 face of the leaf is thickly covered" (Darwin, 1875, 2nd ed., p. 271). 

 The secretion of mucilage continues after removal and Darwin 

 found that when a plant is kept under a bell glass to prevent evap- 

 oration the secretion is produced in such quantities as to run down 

 the leaf surface in droplets; and further that the secretion shows an 

 acid reaction. Goebel found that among the possible acids pres- 

 ent formic acid is one, and believed that this is effective in pre- 

 venting bacterial action. Emanating from these glands, probably, is 

 an odor which Goebel likened to that of honey, which would be at- 

 tractive to insects and thus act as a lure. 



In the case of many carnivorous plants "overfeeding" usually re- 

 sults in the damage and death of the leaf wholly or locally, notably in 

 the pitcher plants. This has not been observed to occur in Drosophyl- 

 lum, and may be accounted for by the inhibition of bacterial action as 

 just indicated. 



The sessile glands do not exude a secretion unless stimulated 



