1922] ROBBINS—ROOT TIPS 77 
That the effect of the peptone or autolized yeast is not protective is 
indicated by the fact that a colloidal material like gelatine is not bene- 
ficial, and by the fact that the beneficial action of the yeast does not 
appear in the first period of two weeks, or even in six weeks when the 
excised roots do not have their tips cut off and transferred to fresh 
solutions. The assumption, however, that the seedling root contains 
some substance or substances derived from the grain which are not 
contained in the basic nutrient solution and which are supplied by the 
peptone or autolized yeast would explain: (1) That root tips trans- 
ferred show less feta growth isos those not transferred. The hypo- 
thetical mat fractionated in the transfers, and the transferred 
root is limited in the second and third periods by a deficiency of these 
materials, which can partially be supplied by the peptone and 
autolized yeast. (2) That in the presence of autolized yeast root 
tips transferred show as much growth as those not transferred. 
The hypothetical substances fractionated in the transfers are sup- 
plied by the autolized yeast. (3) The fact that with the cessation of 
growth of the root tip of an excised root there occurs an excessive 
development of secondary roots. The hypothetical materials which 
are not used in the growth of the main root tip are utilized for growth 
by the meristematic tissue of the secondary roots. 
Whatever the cause of the beneficial action of the autolized 
yeast,’ its effect is limited in some way, as is evidenced by the fact 
that the various concentrations of yeast produce no correspondingly 
increased benefits, and the Pfeffer’s solution containing peptone or 
autolized yeast and glucose will not permit continued growth. 
Summary 
1. Corn roots attached to the grain grow much more rapidly 
under sterile conditions in the dark in Pfeffer’s solution plus 2 per 
cent glucose than do root tips detached from the grain. 
k of space precludes a summary at this time of the voluminous literature 
H A T, HANniG, Bo LEY, W: , BACHMAN, LoE ; 
FULMER, NELSON and SHERWOOD, ose s, and others who have dealt previously 
with the problem of the necessity of accessory substances for plant growth, or 
es: 
who have attempted to cultivate single ak or isolated parts of higher plants. Some 
of the more recent papers on this abies not cited in the earlier publication by the 
writer are given at the end of this paper. A review of the subject indicated is planned 
for a later publication. 
