22 BOTANICAL GAZETTE [JANUARY 
were found in which there was sharp rivalry between two or more 
enlarging potential megaspores (figs. 5-9). Sometimes this occurs 
in the same tetrad, occasionally all four enlarging (figs. 5-7); some- 
times two members of the same tetrad enlarge (jigs. 5, 8, 9); and some- 
times one each of two or more tetrads enlarges. Yet in the end, one 
centrally placed spore always gains the ascendency, the others becom- 
ing abortive. 
This apparent plasticity of Marsilia led me to take up a further 
study of it grown in the greenhouse, and under such conditions that 
a very careful record of results could be kept. My object was to see 
what effect varying conditions of light, heat, moisture, etc., might 
have on the production of the two kinds of spores. | 
Even among seed plants the development of the megaspore is by 
no means so uniform as is that of the microspore, even the formation 
of the well-known tetrad being dispensed with in many forms, such 
as Lilium, where the mother cell forms four nuclei but fails to develop 
walls. These nuclei are thought by many to represent four mega- 
spores. We find also that from a single megaspore there may come 
a varying number of cells within the embryo sac, as found by 
CAMPBELL (5) and JOHNSON (15) in Peperomia, where 16 nuclei are 
formed; and by the writer in Ulmus americana (25), where 8 to 16 
nuclei are formed; and while 8 is the usual number for angiosperms, 
some plants fail to form even so many, as was shown by Miss PACE 
in Cypripedium (19), where only four are formed. 
Also, where the row of four megaspores is formed, there is often a 
sharp struggle for the mastery; sometimes the lower one, sometimes 
the upper one, and even one of the middle ones finally functioning 
as the embryo sac. This is well shown in such forms as Diospyros, 
in which Miss HaGue, in an unpublished paper, finds the above 
conditions. Occasionally two megaspores function, forming two 
embryo sacs, as shown by Ernst (10), and by the writer in Ulmus 
(25) and Pinus (unpublished). 
The nuclei within the sac also contend as to which will function 
as the egg, as shown by CHAMBERLAIN (6) in Aster, and by Miss 
OPPERMAN (18), and by the writer (25). In contrast with the above 
varying conditions of the megaspore we find a very constant method 
of development for the microspore, the mother cell nearly always 
