sin J. LUBBOCK — PUTTOBTOLOGTC.VL OBSERVATrOXS. 



375 



ence, tliougli the explanation wluch I would suggest is very simple 

 wlien once stated. I liave not, however, been as yet able to 

 examine a sufficient number of cases to justify me in. doing 

 more than throwing: it out as a suggestion. 



Seen in section the two forms would be as shown in figs. 88 and 

 90. Now if from the form of the pods, or for any other reason, it 

 is an advantage that the seed should be compressed, as iii fig. 90, 

 then, the thickness of the cotyledons remaining the same, it is 

 better that the radicle should be accumbent ; while, on the other 

 hand, in a thicker or globular seed, as in fig. 88, the incumbent 

 arrangement is most convenient. In fact we find that in groups, 

 such as the Arabide®, Avhere the seeds are as a rule compressed, 

 the radicle is almost always accumbent ; while in incumbent 

 groups, such as tlie Sisymbrese, they are, on the contrary, more or 

 less turgid. As an actual e xamplc of an incumbent form I give 

 Iles^eris matronalls (figs. 88 & 89), and of an accumbent, Chei- 

 rantlius Cheiri (figs. 90 & 91). 



Petioles. 



The cotyledons are sometimes sessile, as in Acer (fig. 9), JUp- 

 poplae (fig. li), Ilakea (fig. 21), Clitorea (fig. 27), sometimes 

 supportcd°on petioles, which in many cases, as in Microhma 

 (fig. 29), attain a considerable length. 



Occasionally we meet with both sessile and petioled cotyledons 

 even within the limits of the same genus. For instance, Djlplu- 

 nium Staplnsagrla (fig- 31) has the cotyledons sessile, whde 

 those of D. elafam (fig. 30) have a p -tiole. In Bryonia lacinwsa 

 (fig. 92) the cotyledons are nearly^essile, while those of B, <hoica 



(fig. 93) have petioles. ■ 



There is nothing, so far as I know, in the structui-e of the 

 seed to account for this difference. It is observable, however, 

 that while the cotyledons of Bryonia Uciniosa{?i^. 92) and those 

 of J9. Staplds.gria (fig. 31) are raised by the caulicle Bomewha 

 abo;e the level of the ground, those of Bjyonia dio,ca (hg. 93) 

 and of D. elatum (fig. 30) are attached close to the ground. In 

 fact the 'cotyledons are carried up in both cases, but m ^. laa- 

 niosa and D. StapMsayria by the caulicle or stalk of the plant, 

 in B^dioica and in ^. elatum, on the other hand, by their own 



petioles. , , 



In short we may say that the cotyledons are, as a general rule, 



sessile when they are raised by the growth of the cauhcle, ^hile 



•' 2 II 2 



