422 
Osterhout. — On the Life- Hi story of 
case in question is an abnormality of rare occurrence, which 
has not been thoroughly investigated, and that it is moreover 
quite susceptible of a different interpretation. The above- 
mentioned cases of Nectria and Sclerotinia show that spores 
may fuse and give up their individuality completely, but 
these are exceptional cases. In the majority of cases 
individuality once established is not subsequently surrendered 
as the result of fusion (apart from the case of gametes), as is 
shown by the conidia of Ustilagineae, which after fusing put 
out separate germ-tubes exactly as though no fusion had 
occurred. On the whole, therefore, the assumption that 
physiological independence is first established and subse- 
quently lost, rests on no direct evidence, and finds its chief 
justification in the general consideration that physiological 
and morphological independence usually go hand in hand. 
On the other hand it is possible to assume that the 
spores contained in a single sporangium probably form 
a physiological unit so long as they remain in contact, 
or at least that they do not acquire complete physio- 
logical independence. The whole contents of the spo- 
rangium are so polarized that at one end they produce a 
growing-point, at the other end rhizoids which penetrate to 
the medulla of the mother-plant. The polarity of the 
individual spores is not developed, or at any rate does not 
manifest itself, until they become separated from each other 
so as to germinate separately, which, as stated above, not 
unfrequently happens. The separate spores may be compared 
to a magnet composed of four separate pieces placed end to 
end. As long as they remain in contact there are but two 
principal poles for the whole, but as soon as they are separated, 
each is polarized independently. Or they may be compared 
physiologically to an animal egg in the four-cell stage, each 
of whose cells if separated from the others produces an 
embryo. A still better comparison is furnished by the 
embryo-like structure in Dicyemidae, which develops from an 
egg, but at a certain stage of its development falls into 
separate cells, each of which (with the exception of one) 
