of the terms ‘ Phyllome 5 and ‘ Caulome ! 139 
thorns, etc.), while leaves have not unfrequently a very exten- 
sive and apparently unlimited apical growth (e. g. Lygodium , 
Gleichenia , etc.), this distinction between axis and leaf cannot 
be maintained. Nevertheless, the fact that the leaf is usually 
limited in its apical growth is to be noted as one of the dis- 
tinctive though variable characters of the leaf. 
V. According to the number of layers of meristem which 
give rise to them respectively, a general difference may be 
traced between leaves and lateral axes. Upon the value of 
this evidence it will be best to quote from Warming, who has 
made such fine and extensive observations in this direction \ 
He writes : f It is impossible to separate phyllomes and 
caulomes by constant morphological and genetic characters. 
We have seen in the second part that they arise from the 
same peripheral tissue, but at slightly different depths : the 
leaves spring generally from the first and third layers of the 
periblem, the weaker leaves, such as the bracts in many 
inflorescences, even from the first layer only; stems hardly 
ever originate in the first layer, but most frequently in the 
third or fourth. This character has its importance, and may 
often serve as a criterion for determining the nature of an 
organ of doubtful morphological character .... but of course 
it must not be regarded as an absolute index, which should 
always be decisive. I think we should rather consider it 
as a circumstance which is intimately connected with the 
size of the organs and the space which they require : the 
more vigorous they are, and the more permanent the role 
which they are destined to play, the more space they require 
and the deeper is their origin in the axis ; since caulomes, 
by reason of their biological role, almost always require more 
space and vigour, they also originate at a greater depth.’ 
VI. We may next consider the criteria of relative time 
and place of origin , these being adopted by Hofmeister as 
his basis of distinction of lateral axis, leaf, and hair 2 : he 
1 Recherch.es sur la ramification des Phanerogames, in F orgreningsforhold, French 
resume, p. xvii. 
2 Allgemeine Morphologie, p. 41 1. 
L 2 , 
