Chapter IX —107 — Pinguicula 



pact buds of various sizes (brood-buds) which can reproduce the plant 

 in the following growing season (Hovelacque). 



The entire leaves and peduncles are provided with two kinds of 

 glands, stalked and sessile (jj — lo, ii), densely scattered on the 

 upper surface, with a much smaller number of sessile glands with four- 

 celled capitals on the lower dorsal surface (75 — 2-4). According to 

 Fenner the latter are hydathodic in character for he observed a 

 minute droplet of fluid water, presumably, on each gland. They may 

 safely be excluded from taking part in the capture and digestion of 

 prey. Goebel had thought their secretion to be mucilaginous but 

 this seems not to be the case. All these glands are of epidermal origin 

 (Gressner, 1877; Fenner, 1904). The stalked glands of the upper 

 surface stand on an epidermal cell, the stalk cell displaying a marked 

 entasis, ending in a single short domed cell supporting the capital com- 

 posed of 16 radiating cells. This secretes and supports a globule of 

 stiff mucilage which serves to entrap and smother the prey, which 

 must be small — only small insects are effectively caught — such as 

 aphides, minute flies of various kinds, etc. The sessile glands have a 

 similar structure, but the stalk cell is not cut off from the foundation 

 epidermis cell, and there is no elongation of it. The base of the gland, 

 therefore, lies flush with the general surface. The capital has only 

 eight cells. The sessile glands of the under surface have capitals with 

 only four cells. All these have been described by Fenner. This in- 

 vestigator further adds that some four rows of cells along the very 

 thin leaf margin are also glandular, and that these secrete mucilage. 

 The margin is of only three cells in thickness, a single course of cells 

 being embraced between the two epidermes (75 — 5). It is always 

 curled upwards through about 180 degrees, and this has been inter- 

 preted as an adaptation for conserving the digestive fluids which escape 

 from the glands on stimulation. Fenner believes also that the escape 

 of secretions from the glands is made possible by the occurrence of 

 pores in the cuticle. I have not been able to see them, but treatment 

 with methylene blue proves the easy passage of solutes, for if a leaf is 

 plunged into a solution of medium strength the capitals of the glands 

 are almost immediately and deeply stained. The capitals of the stalked 

 glands are also stained but not so quickly as those of the sessile glands, 

 perhaps because of the presence of mucilage. With regard to the struc- 

 ture of the cells along the margin of the upper surface, I can see no 

 cytological evidence, such as claimed by Fenner, that they are glandu- 

 lar, nor have I seen a band of mucilage as described by him. J. R. 

 Green (1899, p. 214) cites Darwin to the effect that Pinguicula 

 secretes a digestive fluid on the edges of the upper surface of the leaf 

 which folds over to enclose its captive. On perusing Darwin's account 

 I am unable to subscribe to Green's statement. True, Darwin does 

 use the expression ''placed among one margin" or "on one margin" but 

 this was not meant to indicate that when secretion occurred it was 

 confined to the margin, but that the nearby stalked glands contributed. 

 Drops of meat infusions could not be confined to the margin without 

 coming into contact with nearby glands. Darwin in his first set of 

 experiments was concerned with the possibility of leaf movement which 

 he demonstrated to his own satisfaction. In his experiment on di- 



