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A TEXTBOOK OF THEORETICAL BOTANY 



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23 



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Fig. ii6i. — Various types of stamens. (i6) Parietaria. (17) Hydrocharis. (18) Triticuin. 

 (19) Aconitum. (20) Magnolia. (21) Asimina. (22) TiVw. (23) Elaeocarpus. (24) Meni- 

 spevmiim. (25) Orotitimv. (26) Loasa. (27) Pinguicula. (28) Alchemilla. (29) Rhodo- 

 dendiun. (30) Vacciniiini. {After Le Maoiit and Decaisue.) 



sometimes rather an expression of systematic relationship, where a certain 

 form is constantly found in the same family. 



Examples of specialization in direct relationship to a pollination mecha- 

 nism are so numerous that we can only select one or two striking cases for 

 mention. In Asclepias (Fig. 1162) the filaments are flattened and cohere to 

 form a tube around the ovary. The anthers have only two pollen-sacs, 

 opening longitudinally and each containing a pollinium or mass of united 

 pollen grains with a viscous covering. The anthers are united by their 

 internal faces to the edge of the stigma, with only a narrow slit between each 

 two. From the back of each stamen arises a hood-like structure, or cucuUus, 

 brightly coloured and enclosing a horn-shaped nectary, which projects from 

 the top of the hood. The stigma is much enlarged and has five, marginal 

 ^\^-shaped appendages, one between each two anthers, which are called 

 retinacula. The upper end is a horny clamp called the corpusculum, from 

 which depend two divergent retinacula with adhesive glands which become 

 attached to two poUinia, one from each of two neighbouring anthers. 

 Nectar accumulates in the staminal cuculli and an insect seeking to get at 

 this may catch a leg in the clamp-like corpusculum. On leaving, it withdraws 

 the corpusculum with its two dependent pollinia and carries them to the 

 next flower where they adhere to the glandular lower surface of the stigma. 

 The upper surface is not receptive. 



