ZOOLOGY AND BOTANY, iVIICROSCOPY, ETC. 543 



cell-walls. Their usual form is that of pa2)ill8e. As a rule yellow 

 petals have a stouter structure than those of other colours. 



The colouring-matters of petals may be flivided into two classes — 

 granular, and dissolved in the cell-sap ; the former include, as a rule, 

 green, yellow, and orange, the latter red, blue, and violet petals. These 

 two kinds are sharply differentiated from one another, but are not un- 

 frequently mixed, as in the orange pigment of Colutea cruenta and 

 Fritillaria imperialis, and the brown of the wallflower. A yellow 

 pigment dissolved in the cell-sap is, however, not uncommon. The 

 granular substances, which may be called authoxanthin, are a modifica- 

 tion of chlorophyll ; the soluble pigments, known under the general 

 name of anthocyan, a modification of tannin. These relationships are 

 proved by a number of experiments described in detail. The variation 

 in the colour of petals of the same or of allied species depends on the 

 extent of the metamorphosis of the tannin into the pigment, and on the 

 presence or absence of the yellow colouring-matter. The nature of 

 the chemical change in the transformation of the tannin is probably an 

 oxidation, and it is largely dependent on light. 



As regards the distribution of the pigments ; the general rule, though 

 not without many exceptions, is that anthocyan occurs in the epiderm 

 and veins, the yellow pigment in the lower-lying tissue. Anthocyan 

 has clearly a close relationship to the erythrophyll of leaves. 



Extrafloral Nectaries.*— Herr E. Eathay asserts the existence of 

 extrafloral nectaries in a large number of species of Centaurea and in 

 other genera belonging to the Comjjositae. The purpose of these organs 

 is not the same in all plants. They may exist for the purpose of diges- 

 tion, as in Nepenthes, or for the attraction of insects, such as ants, which 

 would otherwise injure the flowers, as in Impatiens tricornis. They were 

 found to contain a larger or smaller quantity of sugar in all cases except 

 the common peony. Although the insects most commonly found in 

 extra-floral nectaries are ants, yet in sunshine they aj^pear almost always 

 to attract many others, Coleoptera, Hymenoptera, and Dijjtera ; and the 

 term myrmecophilous plants sometimes applied to them is therefore 

 inexact. The sugar-jiroducing parasitic Urediucai, to which the author 

 has already called attention, f appear to benefit the host-plant in the 

 same way as extra-floral nectaries. 



Extrafloral Nectaries of Dioscorea.| — Herr E. C. Correns describes 

 these organs in about twelve species of Dioscorert, those of D. saliva and 

 Batatas having been especially studied. They are wanting in Testu- 

 dinaria elephantipes. These dejiressed glands occur on the under side 

 of the leaf and in the cortical parenchyme of the stem and leaf-stalk. 

 They consist of a mass of cells rich in protoj)lasm and containing a 

 laige nucleus, on the level of the hypodermal layer, their form being 

 ellijisoidal in the leaf, fusiform in the stem and leaf-stalk. The secreting 

 surface, which has no epiderm in a physiological sense, is elliptical or 

 lanceolate, and is covered by a continuous cuticle. The peripheral layer 

 of cells is subcrized in the mature nectary, and therefore represents a 

 protecting sheath. The leptotne of the vascular bundles is connected 

 by transitional cells with the parenchyme-sheath which surrounds the 



* SB. Zool.-Bot. Gesell. Wien, xxxix. (1889) pp. 14-21. Cf. this Journal, cmte, 

 p. 87. t Cf. this Journal, 18S3, p. 246. 



X SI5. Akad. Wiss. Wien, xcvii. (ISSO) pp. 651-74 (1 pi.). 



2 p 2 



