92 MR. MIERS ON THE OUTER FLESHY COVERING OF THE SEED 



quoted (p. 31), as an instance of the progressive formation of the false aril, he describes 

 and figiu-es (in plate 2. figs. 7 & 8) the ovule of Clusia Jlaca, which he examined after the 

 fall of the corolla, at which period he observed that the primine adjoining the fimicle 

 became enveloped for a quarter of its length by a cup- shaped arilliform process, and there 

 can be little doubt that this expands into the entire fleshy coating, which at a later 

 period we know envelopes the testa in the manner I have described in the Clusiece. 



M. Dutrochet, a very able physiological botanist, in discussing the nature of the aril, 

 denies that its growth proceeds immediately from the placenta, and declares his convic- 

 tion, formed after many years of patient investigation of the subject, in the following 

 words : — " L'ojjiaion qui me parait aujourd'hui la plus conforme a 1' observation, est que 

 I'arille est une extension de la partie inf^rieure ou de I'enveloppe corticale du funicule*." 



Mirbel has furnished us with evidence of great importance in the solution of this 

 inquiry : he minutely describes the growth and develoi^ment of the ovule of Cuctimis 

 Anguria, in which he observed, after the period of its fertilization, the prodiiction of two 

 distinct layers of cellular tissue over the primine, originating, no doubt, from an emana- 

 tion and extension of the placentary funicle. His words are, " deux couches de tissu 

 cellulau'c, qui n'ajipartient pas primitivement a I'ovule, mais qui s'appUque a sa sm-face 

 et finit par lui servir d'enveloppe comme ses teguments projjresf," thus affording un- 

 questionable evidence of the extraneous growth of the arUline, evidence since confii-med 

 by the observations of Gasparini and Planchon. I shall be able to show that the crusta- 

 ceous timic of the seeds of the Cucurbitacew, hitherto held to be the testa, as well as its 

 immediate soft envelope, are both of arilliform origin, and that the true integuments 

 resulting from the primine and secundine of the OYvle are to be found in the membrane 

 that immediately invests the embryo, and which has always been considered the tegmen : 

 this fact is attended by some cm-ious phsenomena which will be detailed in another place, 

 where I will adduce many instances of the anomalous development of the raphe. I will, 

 however, here allude to the peculiar structure of the outer tunics iu many of the Cucur- 

 bltacece : if the seeds of Citrullus, for example, be macerated iu water, the outer shell 

 will be found to consist of three distinct parts, which may be termed the epiderm, the 

 mesoderm, and the endoderm. The epiderm is pellicular, transparent, and under the 

 lens is seen marked by large areolar reticulations ; the mesoderm is always fleshy, and 

 composed of parenchymatous, frequently mixed with pleurenchymatous deposits; the 

 endoderm, hitherto considered as the testa, is crustaceous, and consists of transverse hoUow 

 cylinders closely comjjacted and agglutinated together. The epiderm is a continuation, 

 and a dilated termination of the external sheath of the umbilical cord ; the mesoderm is 

 also a continuation of the pleurenchymatous fibres of the funicle, where they serve to 

 protect the more deKcate spiral vessels of the raphe ; but the crustaceous endoderm is a 

 distinct deposit within the vesicle of the epiderm, forming a compressed and indurated 

 sac, enclosing the seed, and always open at its mouth (the hilum), -within which is a 

 space filled with loose cellular tissue, which is also generally extended over the internal 

 surface of the endoderm. These three deposits must be formed at a period subsequent to 

 the fertilization of the ovule, as will be evident from the copious details, illustrated by 



* Mem. Mus. Tiii. 2/3. t Mt'm. .\catl. Paris, ix. 622. tab. 1. fig. 10 e; fig. II h. 





