THE ANGIOSPERMAE 1483 



Loranthaceae, in which, indeed, there may be no proper ovules, cannot 

 form true seeds, and the pericarp is, here also, the chief protective covering, 

 though both embryo and endosperm may be cuticularized. 



The external form of seeds depends on the size and form of the ovary. 

 If they develop in a roomy loculus without overcrowding, they tend to be 

 spherical, but narrow carpels or crowded ovules result in flattened or 

 otherwise distorted seed forms, especially if the seeds are large. The size 

 of seeds is also largely a resultant of their conditions of growth. In plants 

 with very numerous seeds they are generally small {Hypericum, Jasione, 

 Digitalis, Orchis) ; the less numerous seeds of Papilionaceae are large, while 

 the largest seeds are usually solitary. The extreme in this direction is 

 reached in the Palms, such as Cocos, and the gigantic seed of Lodoicea, 

 which weighs up to 15 kilogrammes, is the largest of all. 



The coloration of seeds is occasionally brilliant, either uniformly or in 

 patterns. Whether this has any relation to dispersal by birds or other 

 animals is, in most cases, doubtful. Information on this point is scanty and 

 has been sometimes supplemented by imagination, but undoubtedly many 

 highly coloured seeds are either inedible or are easily destroyed in the crops 

 of birds or are digested by mammals. 



Although a number of dimorphic fruits are known, very few plants have 

 dimorphic seeds. A good example is Plantago coronopiis, the capsules of 

 which contain four large seeds, one in each loculus, and a single, small seed, 

 of different shape, borne, in one loculus only, near the top of the central 

 placenta. When the capsule dehisces the small seed is carried away with the 

 detached lid of the capsule. The small seeds difi^er from the large seeds in 

 their capacity for prolonged flotation, in the small amount of epidermal 

 mucilage formed and in their delayed germination. These characters 

 suggest a difference in biological function as well as in morphology. 



Within the testa, the seed contains the embryo plant, generally in a 

 dormant condition, and the endosperm tissue. In a number of seeds there 

 is also a perisperm, a nutritive tissue formed by a development of the 

 nucellus. If neither of these two tissues is present, the embryo, usually 

 with enlarged cotyledons, fills the whole seed. The heavy thickening of the 

 cell walls in the testa and the large quantity of solid food reserves deposited 

 in the nutritive tissues imply that there is a relatively small water content 

 and in many seeds it amounts to less than 10 per cent, of the whole weight. 



The simple histological structure of the integuments in the young ovule 

 presents a striking contrast to the complexities found in the ripened testa. 

 Little change is visible until the development of the embryo is well advanced, 

 but there may be a considerable amount of breakdown and absorption, 

 which affects not only the nucellus but the inner portions of the inner 

 integument. In some cases the whole of the inner integument and part of 

 the outer integument as well may disappear. Thus the testa may be 

 developed from only a part of the original integuments. 



Where only one ovular integument is present it is generally absorbed 

 in part, so that the testa only consists of the remainder of the tissues and the 



