170 INVESTIGATION OF FOSSIL PLANTS. 



cate with precision their position, the form of their base, 

 and sometimes also their probable direction: we can tell 

 whether they were opposite or verticillate, alternate or spi- 

 rally disposed, deciduous or persistent, imbricated or remote, 

 all characters of great use as means of discrimination, and 

 as frequently aifording important negative evidence on 

 doubtful points. 



Care must be taken not to mistake, for different species, 

 those stems which still retain their cortical integument, as 

 distinguished from such as have lost it. In the two cases 

 the appearance of the scars will be different ; those of the 

 former being more rounded, broader, and more deeply fur- 

 rowed than the latter; for the one is a real scar, showing 

 the outline of the base of the leaf, while the latter is solely 

 caused by the passage of vessels out of the stem into the 

 petiole or footstalk. 



In leaves we can rarely recognise, in a fossil state, more 

 than their mode of venation, division, arrangement, and out- 

 line, to which are sometimes added their texture and surface. 

 All these are of importance, but in unequal degrees. The 

 distribution of the veins, taken together with the mode of 

 division of a leaf, affords evidence of the highest value. If 

 the veins are all parallel, unbranched, or only connected by 

 little transverse bars, and the leaves undivided, the plant 

 was probably monocotyledonous ; and if the veins of such a 

 leaf, instead of running side by side, from the base to the 

 apex, diverge from the mid-ribs, and lose themselves in the 

 margin, forming a close series of double curves, the plant 

 was certainly analogous to what are now called scitaminete, 

 or marantacece, or mumcece ; but supposing that the parallel 

 arrangement of simple veins is combined with a pinnated 

 foliage, then the plant would probably have belonged to 

 cycadece, that curious tribe which stands on the very limits 

 of monocotyledonous and dicotyledonous, and of flowering 

 and flowerless plants. 



If the veins are all of equal thickness, and dichotomous, 

 we have an indication of the fern tribe, which is seldom 

 deceptive. Nevertheless, it must be remembered that the 

 flabelliform leaves, both of monocotyledons and dicotyledons, 

 have occasionally this kind of venation. Even if the veins 

 are not dichotomous, if they are all of nearly equal thick- 



