THE ANGIOSPERMAE 



1385 



Metachlamydeae generally and a few Archichlamydeae have a very small 

 nucellus, which may be no more than a single layer of cells around the 

 embryo sac, and it may disintegrate and disappear at a rather early stage. 



Fig. 1286. — \, Menyanthes trifoliata. B, Fraxinus excelsior. Funicular vascular 

 bundles prolonged nearly to the micropyle. (After Billings.) 



This condition is called tenuinucellate. The majority of crassinucellate 

 ovules are bitegminous and the majority of tenuinucellate ovules are uniteg- 

 minous. The crassinucellate type was called by Warming " eusporangiate " 

 and the other type " leptosporangiate ". As these terms have a definite 

 systematic significance in the Filicales, which is lacking here, they are best 

 dropped, although morphologically they may be justifiable (see also p. 1395). 



The tenuinucellate type may be regarded as reduced, and in some 

 families reduction has proceeded to an extreme. Many of the plants 

 which were formerly held to have naked ovules have been found, on 

 closer examination, to have a single integument which had been mistaken 

 for the nucellus, which has disappeared. For example, in the Rubiaceae 

 a series of stages in the reduction of the nucellus can be traced, ending 

 up with its suppression. In some genera of this family, e.g., Phyllis, a 

 tenuinucellate condition exists, the nucellus being partly sunk in the tissues 

 at the base of the ovule and consisting only of a one-layered skin ot cells 

 over the archesporium. In Boiivardia the archesporium is still more deeply 

 sunken and the nucellus is merely a cap of four or five cells over the top of 

 it. In Oldenlandia the cap has been reduced to a single cell, while finally 

 in Houstonia it has vanished altogether and the integument has closed over 

 the archesporium, completely engulfing it. 



In the parasitic families of Loranthaceae and Balanophoraceae, things 

 have gone much further and the ovules themselves have been suppressed. 

 As in the above case of the Rubiaceae, the work of Fagerlind has shown the 



