228 STUDIES IN PLANT REGENERATION 
e Bermuda. The “shaving brush," as the alga is popularly called, 
is a siphonaceous form in which stalk and head are made up of. 
branching filaments without cross walls. Тһе stalk is encrusted 
with lime, but the filaments of the head are soft and flexible. 
Experiment 49. — The “heads” of a number of these algae 
which were growing in the sand on the bottom of the harbor were 
cut off, and the algae were marked for identification. After 38 days 
it was found that a new head of loose filaments had begun to form 
(Fig. 13). On microscopic examination it appeared that the cut 
ends had closed over and the filaments 
had gone on branching in the normal 
manner. This alga, then, adds another 
to the list of the Siphoneae which have 
a considerable power of regeneration. 
The position and character of the new 
organs which appear on a part in regen- 
eration have been accounted for in a 
great many different ways. Some of 
A ри bet сан these influences аге to be found in the 
cut off a new one was formed. €Xternal conditions to which the part is 
subjected, others in qualities and ten- 
dencies inherent in the part before injury. Though it has gen- 
erally been assumed that an available food supply, either as 
stored nutriment or as the product of the photosynthetic activity 
of the cutting, is a sine gua non of regeneration, few attempts have 
been made to confirm this fact by experiment. The necessity of 
food has, indeed, been recently denied by McCallum.* He found 
that pieces only 8 mm. long of the stems of bean seedlings from 
which the cotyledons had been removed, could, even when kept 
in the dark, develop the buds present in the axils of cotyledons ; 
and Morgan, calling attention to the similar phenomena described 
in regard to animals, has declared that the parts may regenerate 
even under conditions of starvation. 
The following experiments performed on various parts have 
led to a different conclusion. 
Experiment 50. — A plant of Begonia Rex was kept in the 
dark for two days. Four leaves from this plant were then cut in- 
“ 
Peniel? 
* McCallum, W. B. Regeneration of Plants. Bot. Gaz. до: 105. 1905. 
