MORPHOLOGY OF HIGHER PLANTS. 427 



duplicate, or they lie so that the back of one is against the hypo- 

 cotyl, as in Lepidium, which position is knov/n as ixcumrkxt. 



Externally, the seed-coats vary considerably; they may be 

 nearly smooth, as in riciniis ; finely pitted, as in the mustards ; 

 prominently reticulate, as in staphisagria; hairy, as in cotton, 

 strophanthus, and apocynum (Fig. 251) ; or winged, as in the seeds 

 of the catalpa. There are also a number of other appendages, these 

 having received special names: the wart-like development at the 

 micropyle or hilum of some seeds, as in castor-bean and violet, is 

 known as the caruncle ; in the case of sanguinaria, a wing-like 

 development extends along the raphe, and this is knovvn as the 

 strophiole; in some cases the appendage may completely envelop 

 the seed, when it is termed an arillus ; when such an envelope 

 arises at or near the micropyle of the seed, as the mace in nutmeg, 

 it is known as a " false arillus," or arillode. 



Seed Dispersal. — Seeds and fruits are distributed in various 

 ways, and so are often found growing in localities far from their 

 native habitat. In some instances seeds are adapted for distribu- 

 tion by the wind, being winged, as in Patdownia, Catalpa, and 

 Bignonia, or plumed and awned, as in Strophanthus Komhc, 

 Asclepias, and Apocynimi (Fig. 251 ). As examples of fruits hav- 

 ing special parts which aid in their distribution may be mentioned 

 the achene of Taraxacum which is provided with a pappus (Fig. 

 22y), the bladder-like pericarp of Chenopodium, and the winged 

 fruit or samara of maple. The hooked or barbed appendages on 

 some fruits serve to attach them to animals, and thus they may be 

 widely distributed, as in the burdock and Spanish needles {Bidcns 

 bipinnata). In still other cases fruits may be carried long distances 

 by water currents, or even by ocean currents, as those of the 

 Double-cocoanut palm (Lodo'icea scchcUaruui) , which while 

 native of the Seychelles Islands is now found on many of the 

 islands in the Pacific and Indian Oceans. It may also be mentioned 

 in this connection that a number of fruits, as the garden balsam, 

 castor-oil plant, violets (pansy, etc.), IVistaria, etc., are elastically 

 dehiscent and discharge the seeds with considerable force. 



THE INNER STRUCTURE OF THE SEED. 

 The seed-coat usually consists of from two to six layers of 

 cells: (i) an outer layer or so-called epidermis, (2) a layer of 



