THE PAST HISTORY OF MONOCOTYLEDONS 22/ 



3. There is no indication that cither subclass was derived from 



the other. 



4. There are some structural indications that the monocoty- 



ledons must rank lower than the dicotyledons. 



5. The vegetative rank of most cotyledons is so nearly the 



same as to have left no vestiges on the young plant, which 

 is itself vegetative. 



6. The groups into which dicotyledons and monocotyledons 



are divided are " flower subdivisions " of a greatly multiplied 

 rather common vegetative structure. Therefore, we may 

 not expect to find upon the embryo or immature plant any 

 vestigial record of their origin. 



In discussing the results derived from morphology, Bessey, in 

 speaking of tissue systems, draws attention to the short life of 

 monocotyledons and the long life (often amounting to hundreds of 

 years) of dicotyledons. But the ephemeral type of the former occurs 

 amongst the latter, which indicates, he suggests, the close relation- 

 ships of the two subclasses. 



He points out that the young stems of monocotyledons and 

 dicotyledons differ less than old stems, and remarks that " the 

 important difference between these two types of stem is that the 

 skeletal tissues continue to form a single solid column in the dicoty- 

 ledons while they do not in the monocotyledons," and the further 

 fusion of bundles in the former, due to cambial activity, marks them 

 out as a higher group. 



From the normal type of stem seen in lilies, naiads, and orchids, 

 there are two departures in the stem with elongated internodes of 

 grasses and sedges, and the shorter internodes of palms and screw- 

 pines. As to their leaves he says : — 



" The leaves of monocotyledons are usually entire, elongated, 

 parallel-veined blades, placed alternately or scattered upon the 

 stem, to which they are attached directly (in sessile leaves) or 

 indirectly (in petioled leaves) by a common broad base which is 

 rarely supplied with stipules." 



When contrasting these with dicotyledonous leaves which are 

 dentate or lobed, usually broad, net-vemed, opposite and alternate, 

 attached by a narrow base, commonly supplied with stipules, these 

 differences are said to be " due to differences in development." 

 The parallel veins of monocotyledonous leaves are due to localisation 

 of growth at the base of the blade or indefinite bands on each side 

 of its axis. The broad basal attachment of the monocotyledonous 

 leaf may be due to the looser disposition of the fibro-vascular bundles 

 in the stems, or perhaps may be owing to the fact that they are not 

 so far advanced as the leaves of dicotyledons, there being an 

 articulation at the base of the latter only. In the monocotyledons 

 the common leaf type is especially modified in the Graminese. 



The flower is held to consist of a strobilus of "flower-leaves" 

 consisting of sterile leaves and spore-bearing leaves, the former being 

 derived according to Bower, from the latter. The perianth as a 



