58 University of Cdlifoniid I'uhllcaft'nuft in Bofroiij [Vol.7 



2. Tile Mclivo season in T. sessile vai-. <ji<j(iitUtiin lasts fi'oiu Feb- 

 ruary 1o July. In the vicinity of San Francisco Bay, however, unfa- 

 vorable atmospheric and soil conditions cause a majority of plants to 

 end their active season prematurely. On the other hand, in T. ovatum 

 the active season is greatly i)rolonged. The former species ripens its 

 fruit in July, the latter in ]\Iay. 



3. During the active season the young stem-bud, or buds for the 

 next year whose activity had been suspended late in the previous 

 active season, begin to grow anew and differentiate various parts. At 

 the same time additional stem-buds are laid down of which some may 

 develop into buds for the next season. Tliose laid down last, however, 

 and therefore least differentiated, are checked in May, so that they 

 cannot develop farther until the next active season arrives. 



4. In T. sessile var. giganteum a plate consisting of four rows of 

 archesporial cells is cut off in each locule of the anther in May. Late 

 in August or in September the tapetum is cut off. 



5. Tlie dormant season in T. sessile var. giganteum lasts from 

 August to January inclusive. However, activity is renewed in the 

 underground organs when the rainy season begins, in September or 

 October. 



6. The pollen mother-cells divide after the parts of the plant have 

 begun to show renewed activity in growth. If a plant has several 

 stem-buds for next season their pollen mother-cells divide at different 

 times. Other causes affect the time when the meiotic divisions occur, 

 but it may be said that in general the pollen mother-cells of T. sessile 

 var. giganteum divide in October and November. The various phases 

 of the two divisions, including synapsis, occupy not more than three 

 weeks, and the pollen grains are cutting off generative nuclei in three 

 weeks after the completion of the homotypic division. ^Mature pollen 

 may be found in November and December. From that time onward 

 no changes are taking place in the anthers except the development of 

 the mechanism of dehiscence. In many pollen mother-cells the nuclei 

 degenerate in synapsis or before, and many pollen grains fail to 

 develop properly. In T. ovatum the pollen mother-cells divide in 

 August and September and practically all i)ollen is mature by 

 November 1. 



7. The megaspore mother-cells are differentiated just before the 

 microspore mother-cells divide and change but little in appearance in 

 T. sessile var. giganteum before the young shoot appears above ground. 

 The megaspore mother-cell divides and the female gametophyte de- 



