430 FOURTH GROUP.— SEED-PLANTS. 



usually clothed with a distinctly differentiated epidermis, on the configuration of which 

 it depends whether the seed has a smooth appearance as in beans and peas, or shows 

 a variety of sculpturings, such as pits, warts, ridges and the like, as in Datura, 

 Hyoscyamus, Papaver, Nigel/a ; the epidermal cells not unfrequently develope into 

 hairs ; cotton, for example, consists of the long woolly hairs which clothe the seeds of 

 Gossypium ; only a penicillate tuft of long hairs is developed in some cases, as in 

 Asclepias syriaca. The epidermal cells of many seeds, as in Plantago psyllium, 

 P. arenaria, P. Cynops, Lifiuin iisitatissiinuni and Cydonia vulgaris, contain layers of 

 cell-wall which have become converted into mucilage, and swelling up strongly in 

 water envelope the moistened seeds in a layer of mucilage. Pericarps which do not open 

 and which contain small seeds often assume the character of the seed-coat of the seeds 

 of dehiscing fruits ; this is especially the case in the achene and caryopsis, which are 

 therefore popularly called seeds. The circle of hairs, which serves as an apparatus of 

 flight for the dissemination of many seeds of dehiscent fruits, is developed in many 

 achenes as an appendage of the pericarp, as, for example, the pappus of the Compositae, 

 which really occupies the place of the superior calyx. The wings answering the same 

 purpose, which are a development of the seed-coat of some seeds of dehiscent fruits and 

 are well shown in Bigiwnia, occur again on the pericarp of indehiscent fruits, as in Acer. 

 The mucilaginous epidermis of the seeds of dehiscent fruits mentioned above is seen 

 in the epidermis of the mericarps of Salvia and other Labiatae. These and a number of 

 other facts show that in the formation of the pericarp and seed-coat the chief object is to 

 provide the greatest variety of methods for the dissemination of seeds, and in carrying 

 this object into effect structures which are quite distinct morphologically attain the same 

 physiological development, and those which are morphologically similar are very 

 variously developed from the physiological point of view. 



To complete the account of the terminology it should be remarked in conclusion 

 that the spot at which the seed is detached from the funicle, and which is generally easy 

 to be seen in the seeds of dehiscent fruits, is termed the hilutn or umbilicus. The 

 micropyle also is often to be distinguished, lying in anatropous and campylotropous 

 seeds close to the hilum (Corydalis, Faba, Phaseolus), usually in the form of a wart 

 with a depression in the middle. Outgrowths on seeds along the raphe, as in Chelido- 

 nium tnajus, Asarum, Viola, or as a cushion covering the micropyle, as in Euphorbia, 

 are termed crests, strophioles or caruncles. The aril, which wraps the base of the 

 ripe seed or the entire seed in a fleshy succulent envelope, and is easily detached from 

 the true firm seed-coat, has been already mentioned ^. 



A. MONOCOTYLEDONS. 



The seed usually contains a strongly developed endosperm and a comparatively 

 small embryo, the disproportion being very remarkable in the large seeds of Cocas, 

 Phoenix, Phyielephas, Crinum and some others ; in the Naiadaceae, Juncagineae, and 

 Alismaceae the endosperm is absorbed before the formation of tissue in the embryo- 

 sac ; in the Orchideae it is wanting from the first, and in the Scitamineae where it is 

 also wanting, it is replaced by copious perisperm. 



The embryo is usually straight and cylindric or conical, sometimes considerably 

 elongated and then spirally coiled, as in Potamogeton and Zannichellia, sometimes 

 conical or obconical owing to the thickening of the cotyledon at the upper end. The 

 axis of the embryo is usually very short, and small in proportion to the cotyledonary 



' [By the term aril should be designated, as Baillon has pointed out, all growths on the 

 outside of the seed-coat, whatever be their form and position ; the hairs mentioned above, the crests, 

 strophioles, caruncles, etc., are all different kinds of aril.] 



