528 Boodle . — Comparative Anatomy of the 
If this were to take place the ontogeny would give no 
clue to reduction. On the other hand a correlation in 
development, that is to say a tendency to uniformity of 
structure at all nodes 1 or in all parts of the stem, may 
cause the appearance of certain tissues precociously (i. e. at 
too low a level in the seedling-stem). Thus it is possible 
that in plants, where the possession of internal phloem is 
a well-established character in the mature stem, the internal 
phloem may spread downwards below its proper ontogenetic 
level. If this were to take place, all deductions from the 
seedling as to phylogenetic priority of internal phloem as 
compared with a pith would be quite unreliable 2 . To turn to 
the Dicotyledons for an illustration, the presence of internal 
phloem in the pith of the primary root of Asclepias obtusifolia 
(see Scott and Brebner, ’ 90 , p. 272) is almost certainly due to 
a downward extension (speaking metaphorically) of the inner 
phloem of the hypocotyl, and it is not an improbable assump- 
tion that the internal phloem in the lower part of the hypocotyl 
itself has originated in a similar way ; otherwise, on onto- 
genetic grounds, one would have to assume that the ancestry 
of the plant in question did not include forms devoid of inner 
phloem. 
Having pointed out one or two reasons for doubting the 
value of evidence derived from the stem-structure of the young 
plant, it will be as well to state what class of data appear 
to the writer to be important in elucidating a problem like 
that presented by Schizaea . The following is the method 
suggested : — 
1. Any variations in the different parts of the mature stem 
or in the stems of different individuals should be noted. 
2. Special attention should be paid to the structure of the 
node (because complications of advance or reversion, or more 
correctly retension, are to be sought here). 
1 Some features in the seedlings of some plants seem to point to the existence of 
such correlation. 
2 The writer does not wish to imply that in every case pith preceded internal 
phloem. Uniformity in this respect in different phyla is perhaps improbable 
on general grounds. 
