Feb., 1918] Sexual Dimorphism 121 



being a trimerous, pentacyclic structure. In the sedges the 

 leaves are still in three spirals, but in the grasses they are in 

 two; in other words, the leaves of the grasses are two-ranked. 

 During the vegetative growth of Bromus, therefore, its bud gives, 

 off alternately incepts of leaves on opposite sides of the stem. 

 These leaves are differentiated through the activity of hereditary 

 units into the form characteristic of the species. This two- 

 ranked arrangement is a culmination type of morphological 

 expression. As stated, the species evolved from ancestors 

 in which three spiral incepts were successfully organized instead 

 of two. Now the cells of the terminal bud actually contain the 

 hereditary ability to develop threes as will appear below. 

 But for some cause the ability is suppressed. The bud, using 

 a metaphor, dances a two-step instead of a waltz. When a 

 spikelet begins to develop, the two-ranked condition continues 

 and the two-ranked empty glumes are produced and a number 

 of flowers, also in two ranks. The flower bud on the spikelet 

 also produces two-ranked glumes, the flowering glumes, unless 

 the palet represents more than one leaf. But the first set of 

 floral organs proper, the lodicules are produced in a three spiral 

 arrangement. The third, or posterior one, however, is sup- 

 pressed in harmony with the bilateral nature of the evolved 

 plant. One step down in the evolutionary scale, in the bamboo 

 tribe, the third lodicule is present in the proper position as it 

 should be in a monocotyl flower. In some way the bilateral 

 nature does not act so intensely as in Bromus and the higher 

 grasses in general. The lodicules probably represent a corolla, 

 the calyx being suppressed. Next the flower bud of Bromus 

 not only develops three incepts in typical tripartite arrange- 

 ment, alternate with the lodicules, but these organs pass into 

 that strange state which determines the nature of stamens 

 and male gametophytes with male cells or sperms as the final 

 goal. Now all of this up to the formation of microsporocytes 

 is a matter of vegetative growth. The cells in the stamens have 

 received the same heredity as those which produced leaves, 

 glumes, or lodicules. The three stamens are the only structures 

 which show the typical ancestral morphology in the entire life 

 cycle of the grass under consideration. Next the bud fails to 

 perform and the second, expected set of three stamens does 

 not develop. In many bamboos all the six stamens are present, 

 as in any typical monocotyl that is not specialized too much. 



