280 
ing point that the different groups of plants form a continuous 
series from low to high, but, on the contrary, must start with the 
assumption that the origin of different organs must be sought sepa- 
rately in each large group of plants. For this reason it is very 
difficult to give a definition of leaf which will hold for every group, 
as the leaf of the liverwort is analogous to that of the fern and not 
homologous.” 
It is difficult to harmonize these statements of Goebel, for, on the 
one hand, he says the attainment of stem and leaf is an independ- 
ent process in each large group of plants ; on the other, that it 
is difficult to give a definition of a leaf which will hold for every 
group, because these organs are not homologues throughout the 
different groups but only analogues. If their attainment has come 
about independently in each group, why does he expect to find 
them homologues, or what has the question of homology to do 
with the conception of stem and leaf? 
The writer of the present article was puzzling over this ques- 
tion in the laboratory of Professor Schwendener in the summer of 
1894, and on asking him to explain it received the following ex- 
planation of his idea of stem and leaf. He first said that he did 
not share Professor Goebel’s opinion regarding the idea of stem 
and leaf, for he saw no connection whatever with the question of 
homology. He then explained about as follows: 
Suppose we examine a system consisting of main stem and 
branches or shoots: We find first a main stem which we may 
name I. On this arises another which may be called IJ. It is 
somewhat different from the first, smaller and less complex ; this 
number II bears on it another which we may call III, and so on. 
There isno need so far for another name; this might bea branched 
thallus or a branched stem, the branches being the same in gen- 
_ eral character as the first or main branch, only they diminish in 
rank according to their distance from the main stem, so numbers 
do just as well as words to describe them. But when we come to 
the little outgrowths that appear on these branches then we find 
the need of another word or name. They differ from the stem, _ 
but are all of the same grade or rank, whether they are found on 
number I or II, or any other rank of stem. They generally aris¢ — 
_ from the point of vegetation in acropetalous order and are lateral is 
