CLASSIFICATION OF HIGHER PLANTS 23» 



leaf, end abruptly in contact with the water cells and vessels of 

 the stem. The leaf g-ets only what water these trace cells can 

 catch from the side of the cells of the stem. There is no direst 

 water passag:e from stem to leaf. Correlated with this we find 

 that in all Lycopsidans — Lycopodium, Selaginella, Psilotum, 

 Equisetum, Lepidodeiidron, etc. — the leaf is small and individ- 

 ually of little consequence. Such leaves may be numerous, or 

 may be greatly reduced. In the first case — Lycopodiales — the 

 number makes up for size. In the latter — Psilotum. Equisetum 

 ^the stem takes over the function of photosynthesis. These 

 small leaves when fertile, are further limited to one or a few 

 sporangia. And the sporangia are axillary or nearly so, and as- 

 sociated with the upper side of the leaf. 



Now in the Pteropsida there are for every leaf few or several 

 water conducting cells, or vessels which bend out bodily from the 

 stem into the leaf, carrying- an uninterrupted flow^ of water 

 directly out into every veinlet of the foliar structure. This at 

 once removes the limitation of size in leaves. The giant leaves 

 of tree ferns, palms, bananas and the like now become possible. 

 On such a leaf also, with its great assimilating capacity, spore 

 formation may go on ad libitum, as actually occurs in ferns. In 

 stems whose primary vascular tissues take a tubular form, the 

 vessels which curve out into the leaf leave an actual break or 

 gap in the continuity of vascular tissues in the stem. Above 

 every leaf trace there is an area where medullary cells come 

 more or less directly into contact with cortical cells. Such a 

 break in the cylindrical stele Jeffrey calls a leaf gap. And he 

 calls such stems phyllosiphonie. The Pteropsida are therefore 

 primarily phyllosiphonie and megaphyllous. In Lycopsida gaps 

 in the stem stele occur only in relation to branches. This group 

 is therefore called cladosiphonic and microphylloiis.^ It cannot 

 be doubted that this distinction marks the profoundest biological 

 and evolutionary cleavage in the vascular plants. 



Among Pteropsidans, recent studies of fossil plants have shown 

 remarkably close affinities. The fern alliance merges insensibly 

 into the gymnosperms, and these, anatomically at least, grade off 

 very strikingiy into the angiosperms. In fact, the gap above the 

 gymnosperms is as yet more serious and difficult to bridge than 

 that between the gymnosperms and ferns. One cannot tell at 

 present whether a given fossil stem or leaf i' fern or gymno- 



'Cf. Jeffrey: Anatomy of W'oody Plants, 1917. 



