No. 544] BOTANICAL SOCIETY OF AMEBIC A 



235 



in the Dicotyledones, in accordance with the principles 

 of recapitulation, retention and reversion, we arrive at 

 very different conclusions indeed and of much greater 

 significance from the standpoint of evolution. 



It will save time to take a concrete illustration. In 

 the cross section of a small branch of the oak we find 

 the woody part of the stem, composed of ten alternat- 

 ing^ outstanding and depressed segments, separated 

 from one another by ten large so-called primary medul- 

 lary rays. In accordance with the herbaceous hypothesis 

 of the origin of the woody stem, the projecting segments 

 are supposed to correspond to five originally entirely 

 separate bundles. Further the intervening depressed 

 segments of the cylinder, set off from the others by the 

 ten large rays, are supposed to represent parts of the 

 wood which have been secondarily interpolated through 

 the activity of a so-called interfascicular cambium. If 

 we examine the facts in the light of the general principles 

 of plant evolution, a very different and much sounder 

 conclusion is reached. Let us take only the seedling 

 evidence, for the other accords absolutely with it. In 

 the eotyledonary region of the young stem we find for a 

 number of years a completely continuous woody cylinder, 

 without either large rays or depressed segments. At 

 this level in fact the rays are entirely narrow ones of the 

 gynmospermous type. Both the broad rays and the seg- 

 ments which they delimit appear only later. The so- 

 called large primary rays are in fact formed as a result 

 of the fusion of smaller rays into aggregates around the 

 incoming leaf-traces. The broad rays of the oak are- 

 consequently fusion products and not at all primitive 

 structures. They are clearly related as a storage device 

 for the strands which enter the stem from the leaves. 

 There are two broad rays to each leaf, corresponding to 

 its two strongly developed lateral traces, and since the 

 phyllotaxy of the oak is of the 2/5 type, there are nor- 

 mally and originally five pairs of large rays in the stem. 

 The depression of five of the segments delimited by the 



