THE OEIGIN OF MONOCOTYLEDONS. 



293 



into the flowers, we usually get one with fours, as in Fuchsias, privet,' 

 &c. If, however, internodes be developed between the two leaves of 

 each pair, then the arrangement becomes a spiral, and a line drawn 

 through each leaf is like a corkscrew. Now it will be found that, 

 calling any leaf No. 1, the sixth will stand over it, leaves Nos. 1 to 5 

 constituting a " cycle." Let the spiral line be suppressed like a 

 watch-spring, then the five leaves of a cycle or their representatives as 

 sepals, petals, &c., form a floral " whorl." 



The leaves in pairs follow from Dicotyledons having two opposite 

 cotyledons. ' In Monocotyledons, 07ie cotyledon is arrested, so that 

 the first leaf usually stands over the position of the lost cotyledon, and 

 the leaves run in two lines only, as seen in grasses, orchids, iris, &c. 

 But Nature has an alternative, and can place three leaves in a circle. 

 This occurs in sedges, and is usual in floral whorls; hence arose the 

 reduction in the number of parts of flowers of Monocotyledons to three 

 in each whorl. The arrest of one cotyledon is still to be seen in some 

 Dicotyledons, as in the aquatic Trapa, the formerly aquatic Ranunculus 

 Ficaria, and in some terrestrial plants, as Buniuni, Corydalis, and 

 Cyclamen. The rudiment of a second cotyledon was first noticed in 

 wheat by Malpighi,''' who regarded it, conjointly with the scutellum 

 or cotyledon as composing an involucre. M. Poiteau, however, con- 

 sidered it to be a rudiment of the second cotyledon, observing: " Does 

 not its insertion immediately opposite to that of the cotyledon indicate 

 that it is the rudiment of a second cotyledon ? I have found it also in 

 the oat ; it is replaced by a scar in the barley ; no vestige of it is seen 

 in the maize nor in many other grasses. ... If grasses approach 

 sedges in habit, leaves, and flowers, they are distinctly removed from 

 them by the embryo, which appears to have very considerable agree- 

 ments with that of Dicotyledons." t 



A word in explanation must be given as to the cause of the 

 "endogenous" character of the tap-root of the monocotyledonous 

 embryo; though that of a dicotyledonous one is, strictly speaking, 

 endogenous as well. They differ in the amount of cortical tissue or 

 periblem, that of Monocotyledons being thicker; the cause of its being 

 so may perhaps be referred to the now hereditary tendency to arrest the 

 axial root. This has presumably induced the periblem tO' take on a 

 greater activity in multiplying its cells, thereby producing a thicker 

 covering to the pericycle which in turn covers the merismatic apex of 

 the plerome. 



In Dicotyledons the apical point of the plerome of the embryo 

 with its pericycle abuts against the dermatogen and so appears to be 

 exogenous. 



Conclusion.' — Space will not admit of further illustrations and 

 proofs; but the reader is referred to my two papers, one in the Journal 



* Anatome Plantarum, torn. 2, p. 10, tab. 5 (1671). 



t Memoire siir VEmhryon des Grammees, des Cyperacees et du Nelumho 

 (1808). 



