ORIGIN OF ENDOOENS FROM EXOGEXS. 493 



As another illustration Sachs figures a transverse section of a 

 shoot of Aloe Serra*, in which the first six leaves are distichous. 

 The position of the following leaf is shifted through an angle of 

 36°, so that the successive leaves at once pass into the | system, 

 the leaf numbered 13 being over the 8th after 2 coils are com- 

 pleted, the spiral from 1 to 15 being from left to right. 



That the tristichous arrangement is also as easily acquired is 

 seen in this being characteristic of Cyperacece, &c, and probably 

 lays the foundation of the ternary arrangement of the flowers 

 of endogens ; consequently the production of a second leaf in the 

 ordinary systems of phyllotaxis may be at 120 D as well as 180°. 

 Indeed this is probably more often the case than 90°, as M. van 

 Tieghem describes it in Asparagus ; so that the \ and \ arrange- 

 ments are very common among Monocotyledons, as in grasses, 

 sedges, orchids, &c.t 



It is interesting to find that Eobert Brown long ago noticed 

 this difference between Dicotyledons and Monocotyledons, that 

 whereas " in Dicotyledonibus ejusdem foliola primaria cum coty- 

 ledonibus alternant : in Gramineis et Iroideis contra, in quibus 

 solummodo e Monocotyledonum classi hanc determinare possu- 

 mus, cotyledoue opponuntur" J. 



"Dicotyledonous" Monocotyledons. — Tamus communis affords 

 a parallel instance to Asparagus. Dutrochet § has shown that 

 the " first leaf " is situated exactly opposite the cotyledon, which 

 remains embedded within the endosperm on germination. He 

 has no hesitation in calling it a second cotyledon. It is very 

 rudimentary, closely applied to the globular epicotyl, has a 

 median nerve, though it is very delicate in texture and trans- 

 lucent. It never turns green, though exposed to light, and dies 



Text-book, 2nd ed. p. 193, fig. 152. I do not think he describes it cor- 

 rectly. 



t I have fully explained how the ordinary system of phyllotaxis has arisen 

 in my papers " On the Variations of the Angular Divergences of the Leaves of 

 Helianthus tuberosus? Trans. Linn. Soc. vol. xxvi. p. 647 ; and " On the Origin 

 of the Prevailing System of Phyllotaxis; ' op. tit. 2nd ser. vol. i. p. 37. See also 

 * Origin of Floral Structures/ chap. ii. p. 7 seqq. 



X Prod. Fl. Nov. Holl. p. 347. 



§ "Observations sur la Forme et la Structure primitives des Embryons 

 Vegetaux," Kouv. Ann. du Mus. iv. 1835. Graf zu Solms-Laubach gives a 



cam m u n 



well showing the development of one cotyledon and the arrest of the other: 

 Bot. Zeit. 1878, Taf. iv. figs. 21-25. 



