APPENDIX.

87

assume when the stem is wounded, appears to afford anevidence that their formation actually does take placefrom above downward. I have before me the stem of aYucca , in which many branches were sawn off from thesurface during the life of the plant, and which had other-wise undergone injuries penetrating as far as the fibrouslayer. On these wounded places over-growths have beenformed, as in a Dicotyledonous stem, in which the fibresrun down from above to the injured part, till they reachthe upper border of the wound, then deviate to each side,run down along the lateral borders, and at some distancebelow the injured spot again approach together from eachside, in a very acute angle. In this way an over-growthis formed above and at the sides of the wound, but isabsent at the lower border. If the fibres grew upwardfrom below, as Mirbel assumed, they should reach thelower margin of the wound, deviate to the side of it, andgradually approach together again above it; the over-growth ought, therefore, to originate at the lower and notthe upper side of the wound. It is to be observed here,that this over-growth is not effected by an increasingthickening of old vascular bundles, existing at the time ofthe injury, as in the Dicotyledons , but is formed by newly-developed bundles, which are entirely separate from thosesubjacent (as in general, also, in the normal growth ofthe stem, the superimposed fibrous layers are not to becompared with the annual rings of the Dicotyledons , butto be regarded as entirely isolated structures), which ismost clearly shown in the direction of the spiral lines inwhich the fibres run, since these spirals alternate to theright and left, in the successive layers, in a manner ana-logous to what is found in Xanthorrhcea.

The analogy between the structure of the stem of aYucca and a Palm is so great, that we are justified indrawing a conclusion from the phenomena we observe inthe former, as to the processes occurring in the latter.The distinction between the two stems lies chiefly inthis, that in Yucca the lower extremities of the vascular