Box.— Vol. I.] CANNON— AVENA FATUA, 33 1 



II. Origin of the Floral Organs. 



The spikelet of Avena branches sympodially, the apex of 

 the young spikelet developing into flowers and finally 

 pistils. Each flower arises below the apex of the pri- 

 mordium of the spikelet and grows at right angles to it, so 

 that the rachilla is sharply angular when young (fig. i). 



The primordium of the spikelet consists of three well 

 defined series of tissues: (i) a dermatogen, (2) a single 

 layer of large hypodermal cells — the periblem — which sur- 

 rounds (3) an axial strand — the plerome cylinder. As the 

 primordium increases in length the cells of the epidermis 

 and of the periblem divide by anticlinal walls and form a 

 continuous cap to the plerome. The plerome arises from 

 a single cell, or perhaps a group of cells, that terminates the 

 axial strand (fig. 3). All of these cells are well filled with 

 protoplasm, are without conspicuous vacuoles, and have 

 relatively large nuclei. 



The floral organs all arise in much the same way upon 

 the periphery of the primordium, beginning with the lower 

 glume and ending with the carpel. Each begins as a ridge 

 of tissue that extends part way around the primordium 

 of the flower. The origin of each floral organ will be 

 spoken of in the order of its appearance. 



The lower glume and the young flower apex appear to 

 arise simultaneously just below the tip of the primordium of 

 the spikelet (fig. 4). At first these two rudiments are very 

 similar, but the flower apex soon becomes broader, and the 

 lower glume somewhat deeper by its growth and by the 

 stretching out of that part of the rachilla that is immediately 

 below it. At this time the glume is a semicircular organ, 

 club-shaped in longitudinal section, composed of a well 

 defined dermatogen enclosing about three layers of cells. 

 As the lower glume increases in length it takes a direction 

 which coincides with the direction of that part of the 

 rachilla from which it springs, but it soon bends toward the 

 tip of the spikelet (figs, i and 2). At that stage in the 

 development of a flower when all of the floral organs are 

 first to be distinguished, a narrow outgrowth appears near 



