Alternation of Generations in the Dictyotacece. 185 
regularly alternate with each other. The cytological evidence 
seems to prove conclusively that they do. If the vegetative cells 
of tetrasporic plants of Dictyota or Padina be compared with similar 
cells of the sexual plants, the dividing nuclei of the former are 
found to have twice as many chromosomes as those of the latter. 
In the stalk-cell division of the tetrasporangium there are thirty-two 
curved, rod-like chromosomes and the division is homotype in 
character, but the next division has a long period of preparation, 
there is a distinct spirem thread and the synapsis which usually 
accompanies the reduction stage, and when the mitotic figure is 
formed there are sixteen chromosomes which exhibit the charac¬ 
teristic forms of the heterotype division. The succeeding mitosis, 
which completes the formation of the four nuclei of the tetra¬ 
sporangium, is of the usual homotype character with the reduced 
number of chromosomes. Mottier 1 has already described the 
reduction division and most of the other mitoses in the asexual 
plant, but he has not examined the cytology of the germinating 
spore or of the sexual plant. When the tetraspore germinates it 
is found that the reduced number persists in all the divisions. 
In the male Dictyota plant the reduced number prevails in all 
the numerous divisions of the antheridium. In the oogonium there 
is only one mitosis, the one that cuts off the stalk-cell; here also 
the chromosomes are sixteen in number. 
If liberated oospheres are left unfertilized they start dividing 
parthenogenetically, but after forming a few cells they invariably 
die. The figures formed in them are multipolar and show all kinds 
of irregularitiesof form, but the chromosomes always show thereduced 
number. The mitotic figures found in fertilized oospores, as might 
have been expected, always have the full number of chromosomes. 
Here then we have a spore-bearing form whose somatic nuclei 
are characterized by the double number of chromosomes, while the 
gamete-bearing plant has the reduced number. The reduction 
stage moreover has all the characters that distinguish this stage in 
the higher plants. The inference is natural that the tetraspore 
germling with its reduced number of chromosomes will grow up 
into a sexual plant, while the fertilized oospore with its double 
number will produce an asexual plant; in other words there is true 
alternation of generations here. 
It 1 'emains to prove this experimentally by cultivation from 
spore to spore. The numerous attempts made to this end have 
hitherto not been successful, owing to the difficulty of getting the 
plants to fruit in cultivation. 
1 Mottier. Ann. of Bot. XiV., 1900. 
