448 The Botanical Gazette. [October, 
experiments except as regulatory phenomena. Try the dif- 
ferent hypotheses of pressure, tension of tissues, relation be- 
tween size of cells and thickness of membrane, and so forth, 
and none of them is satisfactory. 
There is no claim made here that all growth of mechanical 
tissue is regulated by stress. It is influenced probably by 
transpiration, as the researches of Kohl'!? seem to indicate, 
and there is doubtless also an hereditary factor. 
There is yet one feature to be added to the subject of regu- 
latory growth. In my experiments, Vicia faba and Melian- 
thus major, after growing in casts for several weeks, were re- 
leased, and then showed a great constriction at the place 
where the growth had been confined. Within three or four 
weeks this constriction had entirely disappeared. Microscop- 
ical examination showed that, since the removal of the cast, 
there had been in all the plants an excessive development from 
the cambium, in the place of constriction, reaching in one case 40 
per cent. to 50 per cent. more xylem elements in the abnormal 
segment than in the normal parts, above and below it. On 
the removal of the casts the weak segments felt suddenly the 
full weight of the stem. They responded by building a suf- 
ficiency of supporting tissue. There is reason, too, why there 
should have been more than the normal amount formed. 
The plaster was laid about these stems while the pith was 
still expanding, and the vascular zone moving outward from 
the center. The cast checked this movement, and by subse- 
quent development within the rigid envelope, the supporting 
xylem cylinder was fixed nearer the center than normally. 
When released from the confinement, a greater radial thick- 
ness of mechanical tissue was needed in the narrower cylin- 
der than in the normal cylinder to give the same degree of 
strength. 
By the expression ‘‘regulatory growth,” we do not come to 
the actual means or to the specific stimulus for that growth. 
We may say that the plant has the ability to respond to stress, 
but the notion stvess is complex, and will doubtless by future 
research be subdivided. But this much seems certain: The 
formation of such growths as have been recounted in this pa- 
per is no longer to be explained as simple mechanics, but 
rather as a member of the increasing number of phenomena 
of irritability. 
University of Michigan. 
**Kohl, Die Transpiration der Pflanzen. Braunschweig. 1884. 
