PLANTS WITH EXTKAFLOKAL NECT ABIES. 67 



floral nectar glands, which have been noticed by systematic as well 

 as by physiological botanists (Baillon, Miiller Arg., Bentham and 

 Hooker). They are found on the stipules of Jatroplia multifida, and 

 on the petiole at the base of the leaf blade of Aleurites iiioluccan(t. In 

 a paper by Percy Groom on the extrafloral nectaries of the allied 

 Aleurites cordata"- these petiolar nectaries are described as follows: 



Each nectary is a green-stalked shallow basin, the concavity of which is tinted red. 

 The secreting cells which line the basin form a single layer of palisade-like cells. 

 The general cuticle is preserved over these, and the secretion emerges through splits 

 in it. The main body of the basin is composed of an anastomosing system of con- 

 ducting parenchyma and ground parenchyma. * * '* The secreting cells contain 

 proteids, sugar, a red coloring matter (a compound of tannin?), tannin, but no 

 starch. In the ground parenchyma starch, tannin, and crystals of calcic oxalate 

 occur. The conducting parenchyma contains sugar, but no starch or crystals. 

 * * * Darkening the nectaries of leaves on the plant or of excised leaves, or 

 darkening the whole leaves, caused a gradual disappearance of the starch, but the 

 nectaries continued to excrete for a time. 



The above description applies very nearly to the stipulary nectaries 

 of Ricinus, a photograph of a cross section of which, made by Mr. 

 B. J. Howard, of the United States Department of Agriculture, is 

 shown in Plate IX, fig. 1. 



Among the Malvaceae growing in Guam several are provided with 

 nectar glands on the underside of the midrib. These are most con- 

 spicuous in TJrena sinioata, occurring not only on the midrib, but some- 

 times on the main lateral ribs of the palmate leaves. They also occur 

 on all leaves of cotton {Gossypium sp.) and on the midrib of Pariti 

 tiliaceuhi (PI. X, fig. 2), in the form of vaginate glands. A photo- 

 graph of a cross section of the nectar gland of a cotton leaf, also made 

 by Mr. Howard, is shown in Plate X, fig. 1. 



The sweet fluid secreted by these glands is eagerly sought by sugar- 

 loving insects, and a number of authors maintain that the power of 

 secreting it has been specially gained by plants for the sake of attract- 

 ing ants and wasps, which will serve as defenders against caterpillars, 

 leaf -cutting insects, or other enemies; biit Darwin,* after a series of 

 observations, could not see any reason to believe this to be so with the 

 species observed by him, although the fact that these glands are 

 visited by insects for the sake of their nectar can be verified at any 

 time of the day when the sun is shining, and these insects must serve 

 as a protection for them. It is interesting to note that these glands 

 may occur in one species and be absent from another closely allied to 

 it of the same genus. Indeed, there are species in which the glands 

 are present on some leaves and absent from others, and of their vari- 

 ability we have alreadj^ spoken in connection with Ricinus and Urena. 



" Annals of Botany, vol. 8, p. 228, 1894. 



6 Cross and self fertilization, pp. 403, 404, 1877. 



