1246 A TEXTBOOK OF THEORETICAL BOTANY 



is potentially nectariferous. Gunthart has supported this but he maintains 

 that the various forms are attributable to space restriction due to the 

 relative development of the other floral parts. The form of the nectary is 

 therefore a " passive " character in his sense, being dependent on the opera- 

 tion of three " active " characters: the form of the gynoecium, the constric- 

 tion of the calyx and the raising of the median sepal insertion level. The 

 ring-form is apparently the original and the others are regarded as deriva- 

 tive. Because of the dependent status of the nectary form, Gunthart rejects 

 Bayer's classification as artificial. 



We are here treating only of nectaries which are included in the flower, 

 but extra-floral nectaries are not uncommon. They may be formed on 

 leaves, on stipules or on the stem (see Volume I, pp. 505 and 1028), or they 

 mav be formed in close connection with the flower, on the outside of the 

 sepals in Malpighiaceae, on bracts or on the pedicel, in which position they 

 may play a part in pollination analogous to that played by the true floral 

 nectaries. A good example is afforded by Euphorbia, where the nectaries 

 are formed on the margin of the involucre of the cyathium. 



It is first necessary to draw a distinction between nectaries proper and 

 nectar-holders, for, although the two things may sometimes be combined 

 in one structure, e.g., Raminciihis, they are frequently quite separate and 

 indeed one may be present without the other. Nectaries are frequently 

 present without any nectar-holder and, on the other hand, the spurs of 

 some flowers, especially among Orchids, are apparently nectar-holders, 

 although the nectaries have disappeared. This view is supported by the 

 presence of both structures in other Orchids, in which the spurs function 

 as suggested. 



From the point of view of floral biology nectaries may be divided into 

 two types: those with exposed nectar and those with concealed nectar, 

 which is a very important difference with regard to pollination. These 

 types occur, however, irregularly distributed among flowers of all groups 

 so that to classify nectaries it is best to proceed on a topographical basis, 

 according to the parts of the flower in which they are formed, which allows 

 of a much more systematic treatment. (Adapted from Fahn.) 



I. Toral Type. Nectar produced on the surface of the receptacle or 

 from some part of it. 



{a) Marginal. Nectar produced near the base of the sepals and 



collecting in or around them. 



Example: Capparis. 

 (b) Discoid. Nectar produced from a ring or portions of a ring 



around the receptacle. 



Examples: i. Complete ring in Boraginaceae and Labi- 



atae. 



2. Partial ring in Rhinanthaceae. 



3. Ring of small swellings in Cistiis or of 



scales in Crassulaceae. 



