APPENDIX. 87 



assume when the stem is wounded, appears to afford an 

 evidence that their formation actually does take place 

 from above downward. I have before me the stem of a 

 Yucca, in which many branches were sawn off from the 

 surface during the life of the plant, and which had other- 

 wise undergone injuries penetrating as far as the fibrous 

 layer. On these wounded places over-growths have been 

 formed, as in a Dicotyledonous stem, in which the fibres 

 run down from above to the injured part, till they reach 

 the upper border of the wound, then deviate to each side, 

 run down along the lateral borders, and at some distance 

 below the injured spot again approach together from each 

 side, in a very acute angle. In this way an over-growth 

 is formed above and at the sides of the wound, but is 

 absent at the lower border. If the fibres grew upward 

 from below, as Mirbel assumed, they should reach the 

 lower margin of the wound, deviate to the side of it, and 

 gradually approach together again above it ; the over- 

 growth ought, therefore, to originate at the lower and not 

 the upper side of the wound. It is to be observed .here, 

 that this over-growth is not effected by an increasing 

 thickening of old vascular bundles, existing at the time of 

 the injury, as in the Dicotyledons, but is formed by newly- 

 developed bundles, which are entirely separate from those 

 subjacent (as in general, also, in the normal growth of 

 the stem, the superimposed fibrous layers are not to be 

 compared with the annual rings of the Dicotyledons, but 

 to be regarded as entirely isolated structures), which is 

 most clearly shown in the direction of the spiral lines in 

 which the fibres run, since these spirals alternate to the 

 right and left, in the successive layers, in a manner ana- 

 logous to what is found in Xanthorrhoea. 



The analogy between the structure of the stem of a 

 Yucca and a Palm is so great, that we are justified in 

 drawing a conclusion from the phenomena we observe in 

 the former, as to the processes occurring in the latter'. 

 The distinction between the two stems lies chiefly in 

 this, that in Yucca the lower extremities of the vascular 



