STRUCTURE OF FLORAL ORGANS. 541 



depending on the greater or smaller number of layers of colour-cells 

 beneath the epidermis, the tints differing accordingly as cells containing 

 colouring-matter of different hues overlie one another. 



Structure of the Anther, &c. Stamens and pistils are composed 

 of rather regular parenchyma with a delicate epidermis, and fibro- 

 vascular ribs more or less developed in different cases. 



The structure of the anther is somewhat complex, varying not only in 

 different plants but also in different stages of growth. At first consisting 

 of cells of about the same size and form, it subsequently presents a central 

 mass devoted to the formation of the pollen (see under Physiology of 

 Reproductive Organs). This central mass is overlain by three layers of 

 cells : first in order going from within outwards is the endotliecium, con- 

 stituted by a single layer of delicate cells of a different size and shape 

 from the rest, and usually disappearing as the pollen-grains are matured. 

 These cells appareutly contain nitrogenous contents, supposed to be 

 applied to the nutrition of the pollen-cells during their growth. This 

 layer is persistent in the case of anthers opening by pores. Succeeding 

 the endothecium are one or more layers of permanent cells, some of 

 which contain spiral fibres. These cells constitute the mesothecium. The 

 fibrous cells vary in number and situation in different plants, and are 

 sometimes entirely absent, as in the case of anthers opening by pores ; 

 hence they are supposed to act hygrometrically in the dehiscence of 

 valvular anthers. The third layer of the anthers is of an epidermal 

 character, and is called the exothecium. The connective has the general 

 structure of the filament ; sometimes, as in some Lilies, it contains fibrous 

 cells. Each cell of the anther is partly subdivided by cellular projections 

 from the connective; to these processes M. Chatin gives the name of 

 jtlacentoidS) being of opinion that they contribute to the nourishment of 

 the pollen. 



Pistil. The pistils and fruits have, for the most part, the general 

 structure of leaves. Some of their fibro-vascular bundles run along the 

 placentas and give off spiral vessels through the funiculus to terminate at 

 the chalaza of the ovule. The style has usually in the centre a quantity 

 of loosely packed, cylindrical, elongated cells, constituting the conducting 

 tissue for the pollen-tubes. The stigmatic cells are devoid of epidermal 

 covering, and hence present the form of partly detached prominences or 

 papillae. 



Ovules, Seeds. The coats of the ovule, as well as the nucleus, are de- 

 scribed as wholly cellular; frequently, however, the vascular tissue, 

 instead of ceasing at the chalaza, is prolonged upwards into the coats, as 

 may be seen in the testa of many seeds, e. y. Almond, Walnut, Cycas, &c. 

 Great changes take place in the nature and arrangement of the cells as 

 the ovules ripen into the seeds. The disposition of the cells of the outer 

 investment of seeds is often very beautiful and characteristic. 



In the ripening of the fruit the organizing tissues, which carry on the 

 growth of the organ, are situated between the inner epidermal layer and 

 the fibro-vascular zone, as in the case of leaves. IVloreover the fibro- 

 vascular bundles are arranged, as in the leaf-blade, with reference to a 

 surface, and not in a cylindrical disposition as usually in stem-organs. 



