THE PAST HISTORY OF MONOCOTYLEDONS 16$ 



by the junction of the scales uniting to form the pericarp, the 

 micropyles lying in the angles of the furrows. 



In this anomalous cycadean group we have doubtless the first 

 beginnings of the Angiospermic stock. A long gap intervenes 

 between it and the forms regarded as primitive amongst Angiosperms, 

 viz. Ma«;nolia, Liriodendro/i, etc., but there is much to recommend 

 their derivation from Bennettites, which in turn is in the same line as 

 the Pteridosperms. (PI. I. fig. 3.) 



We propose to see what evidence there is in the rocks as to the 

 history of monocotyledons, and also as to their origin. This may 

 lead us to a general origin for Angiosperms. 



2. Generai, Character of Monocotyledons. 



Angiosperms are divided into two groups, monocotyledons and 

 dicotyledons, a division based upon the number of cotyledons or 

 seed leaves, there being only one in the former, two in the latter in 

 the seedling, as commonly understood. 



But this difference, which is not obvious except in the young state, 

 nor always then to be observed, is not the only one. Other differ- 

 ences are correlated with it, and to the main distinction there are 

 exceptions amongst dicotylar forms, where some are monocotylous. 



As a general rule these other characters are : — 



1. One cotyledon. 



2. Scattered bundles of the stem. 



3. Development of the embryo. 



4. Parallel venation of the leaves. 



5. Short duration of primary root. 



6. Albuminous seeds. 



7. Trimerous floral structure. 



Some characters are due to physiological conditions of existence, 

 depending largely upon the adaptation of the plant to its surroundings, 

 whilst others are doubtless due to heredity. 



The first three and the last appear to be largely due to heredity, 

 whilst the others are more or less adaptive characters. And it is 

 doubtless only because they have been longer fixed that the former 

 may now be strictly hereditary. The latter may be homojilastic, 

 occurring in some degree in other groups exposed to similar con- 

 ditions. A feature common to a large proportion of monocotyledons, 

 33 per cent, as against 4 per cent, in dicotyledons, is the prevalence 

 of the aquatic habit. With this many characters are connected and 

 may be said to be adaptations owing to the same physiological cause. 



Although there are so many aquatic monocotyledons, they are 

 distributed over six cohorts out of ten. Amongst Palms, Cyclan- 

 thacese, Scitamineae, and Orchids there are few if any aquatic forms. 

 In dicotyledons they are mainly confined to Polygonales, Ranales, 

 Rosales, Geraniales, Myrtiflorae, Contortae (Gentianaceae), and 

 Tubiflorae. The comparative uniformity of type in monocotyledons 

 may be attributed to their aquatic life. Whilst many forms are 



