DIFFERENT ORGANS. 55 
are to be considered as so many modifications of one 
organ ; viz. the leaf. 
88. In the seed, the two seed-lobes or cotyledons, 
are to the heart or corculum, (which constitutes the 
stem and root of the future plant) two leaf-like organs; 
in some plants, as for example, the Lupin, &c. the 
seed-lobes rise out of the ground, and assume the 
colour and appearance of leaves. 
89. The characters of the leaf have already been 
noticed, and it has been shown that leaves assume two 
principal modes of arrangement; that of being placed 
alternately and also opposite to each other on the 
stem (pl. 18. f. 6. alternate; f. 7. opposite leaves). In 
addition to this it must be observed, that in almost all 
cases in which leaves are placed opposite to each other, 
yet each pair of leaves will be found to be alternate 
with that pair which is above and below it on the stem. 
Ficure 7. represents the stem of the Pimpernel, and 
shows that the middle pair of leaves are placed in a 
cross direction to the pair above and below it; if the 
fourth pair had been produced, it would assume a 
position which would agree with the middle or second, 
but would alternate with the third below it and the 
next above it, 
This is illustrated in f. 8. in which the leaves of the 
great Loose Strife are seen to be placed in threes on 
the stem, the lowermost leaves, a. 6. ¢. are placed 
differently to those immediately above them d. e./f. 
and the third row, g. . 7. will be found to agree with 
the lowest, and the fourth set of leaves, j. k. 7. will 
agree with the position of the second, d.e. f. Thus, 
