THE MORPHOLOGICAL COMPOSITION OF PLANTS. 73 



are at the same time supplied with a rationale of those truths 

 which vegetal morphologists have established. Those homo- 

 logies which Wolff indicated in their chief outlines and 

 Goethe followed out in detail, have a new meaning given to 

 them when we regard the pheenogamic axis as having been 

 evolved in the way described. Forming the modified con 

 ception which we are here led to do, respecting the units of 

 which a flowering plant is composed, we are no longer left 

 without an answer to the question What is an axis? And 

 we are helped to understand the naturalness of those cor 

 respondences which the successive members of each shoot 

 display. Let us glance at the facts from our present stand 

 point. 



The unit of composition of a Pha3nogam, is such portion of 

 a shoot as answers to one of the primordial fronds. This 

 portion is neither one of the foliar appendages nor one of the 

 internodes; but it consists of a foliar appendage together 

 with the preceding internode, including the axillary bud 

 where this is developed. The parts intercepted by the dotted 

 lines in Fig. 123, constitute such a segment; and the true 

 homology is between this and any other foliar organ with the 

 portion of the axis below it. And now observe how, when we 

 take this for the unit of composition, the metamorphoses 

 which the phaenogamic axis displays, are inferable from known 

 laws of development. Embryology teaches us that arrest 



of development shows itself first in the absence of those parts 

 that have arisen latest in the course of evolution; that if 

 defect of nutrition causes an earlier arrest, parts that are of 

 more ancient origin abort; and that the part alone produced 

 when the supply of materials fails near the outset, is the prim 

 ordial part. We must infer, therefore, that in each seg 

 ment of a Phaenogam, the foliar organ, which answers to the 

 primordial frond, will be the most constant element; and 

 that the internode and the axillary bud, will be successively 

 less constant. This we find. Along with a smaller size of 

 foliar surface implying lower nutrition, it is usual to see a 



