30 VITELLUS HILUM. [BOOK j. 



Mirbel has, however, justly remarked that the primine and 

 the secundine are, in the seed, very frequently confounded ; 

 and that, therefore, the word testa is better employed, as one 

 which expresses the outer integument of the seed without 

 reference to its exact origin, which is practically of little im- 

 portance. The tercine is also, no doubt, often absent. He 

 observes that these mixed integuments often give rise to new 

 kinds of tissue ; that in Phaseolus vulgaris the testa consists, 

 indeed, of three distinct layers, but of those the innermost 

 was the primine ; and that the others, which represent nothing 

 that pre-existed in the ovule, have a horny consistence, and 

 are formed of cylindrical cellules, which elongate outwards, 

 in the direction from the centre to the circumference. And 

 this is probably the structure of the testa of many Leguminous 

 plants. 



De Candolle states, that it sometimes happens that the endo- 

 pleura thickens so much as to have the appearance of albumen, 

 as in Cathartocarpus fistula, and that in such a case, it is only 

 to be distinguished from albumen by gradual observation 

 from the ovule to the ripe seed. This is, however, denied by 

 Schleiden. 



One of the innermost integuments is occasionally present 

 in the form of a fleshy sac, interposed between the albumen 

 and the embryo, and enveloping the latter. It is what was 

 called the vitellus by Gsertner, and what Eichard, by a 

 singular prejudice, considered a dilatation of the radicle of 

 the embryo : to his macropodal form of which he referred the 

 embryo of such plants. Instances of this are found in Nym- 

 phsea t and its allies, and also in Gingerworts, Peppers, and 

 Saururus. Brown, who first ascertained the fact, considers 

 this sac to be always of the same nature and origin, and to be 

 the sac of the amnios. 



The end by which the seed is attached to the placenta is 

 called the hilum or umbilicus (Plate VI. fig. 5. c, 17. e, 11. c, 

 &c.) ; it is frequently of a different colour from the rest of the 

 seed, not uncommonly being black. In plants with small 

 seeds it is minute, and recognised with difficulty; but in some 



