235 
Compound Rays in the Lower Dicotyledons, 
trees, vines, and shrubs from this supposedly primitive condition, the fasci- 
cular cambiums of the bundles are extended in a tangential direction through 
the fundamental tissue enclosed between the bundles, or* primary medullary 
rays ’ (see B). The fascicular cambiums are thus joined together, forming 
a cambium ring. The part of the cambium, ic , extending between the 
bundles is called interfascicular cambium in contrast to the cambium of the 
fascicular segments, /V. By the continual division of the cells of the cambium 
ring, secondary xylem is laid down on the inside of the ring and secondary 
phloem on the outside. Thus the primary phloem and xylem are forced 
further and further apart, and growth in diameter takes place. This con- 
dition is illustrated in Q in which p and # are the primary phloem and 
xylem, fh the secondary xylem formed by the fascicular cambium, and if h 
the tissues formed by the interfascicular cambium. It is to be noted that 
no primary xylem subtends the segments ifh. In the primitive condition 
the entire fundamental substance between the bundles is supposed to be kept 
continuous from the pith to the inner bark by the divisions of the interfasci- 
cular cambium. In more specialized forms more or less of the tissue formed 
by the interfascicular cambium is composed of lignified elements or secondary 
xylem, and the ‘ primary rays ’ are thus confined to more or less restricted 
radii of the interfascicular segments. It has also been supposed that 
arborescent and shrubby plants have been evolved from herbaceous forms 
by the gradual fusion of separate fibro-vascular bundles into a compact 
woody cylinder. 
A serious objection to the hypothesis which we have just outlined 
is the fact that, although arborescent and shrubby forms are supposed to 
have been evolved from herbaceous forms, the lowest dicotyledonous plants 
possess well established woody stems, and herbaceous plants occur mainly 
among the higher families of the Dicotyledons. Similarly the arborescent 
or shrubby condition is a distinctive feature of Gymnosperms to the ex- 
clusion of the herbaceous type. If the woody Phanerogams have been 
evolved from herbaceous ancestors the latter must have entirely perished. 
There is, moreover, no undoubted palaeobotanical evidence to indicate 
that such herbaceous progenitors ever existed. In this connexion it is of 
interest to note that the surviving representatives of the aborescent palaeozoic 
Cryptogams are herbaceous or semi-herbaceous in habit. 
Furthermore, in accordance with the Sachsian theory, it would be 
reasonable to expect that in the individual development of the most primi- 
tive living dicotyledonous plants, the earliest formed tissues, which by the 
law of recapitulation reflect the phylogenetic history of the plant, should 
possess anatomical characters resembling ancestral characters more closely 
than do those tissues which occur in the mature portions of the plant. In 
fact, according to the Sachsian hypothesis, in the development of the stem 
of woody dicotyledonous plants, the transition from a separate ring of 
