30 MORPHOLOGY OF ANGIOSPERMS 



numerous. Caldwell 23 lias called attention to the occurrence 

 of what might be regarded a single microsporangium in Lemna ; 

 it is well known that the stamens of Asclepiadaceae produce 

 only two microsporangia ; and in Hamamelis (Shoemaker 35 ) 

 there is a single sporangium to each " pollen-sac." Eight mi- 

 crosporangia had long been observed among the Mimoseae when 

 Engler 9 reported a still larger number. Among the Orchida- 

 ceae Guignard 10 reports eight microsporangia in the stamen 

 of Calanthe veratrifolia ; and among the Onagraceae, as 

 in Gaura, more than four microsporangia are suggested by 

 the pollen-sacs (see Goebel, 11 p. 369, foot-note 2). Among 

 Loranthaceae Van Tieghem 13 says that the number of pollen- 

 sacs is exceedingly variable, ranging from one to an indefinite 

 number ; and the same is true of the Balanophoraceae, as re- 

 ported by several investigators. Attention should be called to 

 the fact, however, that the number of sporogenous masses finally 

 developed may not necessarily determine the number of spo- 

 rangia, for plates of sterile tissue, derived from sporogenous 

 tissue, have been observed to divide a single mass of sporoge- 

 nous tissue into two or more. This has been made out clearly 

 by Caldwell 23 in the case of Lemna (Fig. 11) ; and in those 

 cases in which more than four microsporangia are reported a 

 detailed study of their origin is desirable. In the case of 

 branching stamens, referred to on p. 23, the microsporangia 

 may become very numerous. 



The time for the formation of microsporangia in relation 

 to what is usually called " the growing season " has not re- 

 ceived the attention it deserves. In 1896 Anna Smith 14 re- 

 ported that she had discovered the pollen mother-cells of Tril- 

 lium dividing in the spring beneath frozen soil. In 1897 

 Chamberlain 16 called attention to the fact that the microsporan- 

 gia of Salix glaucophylla are in the mother-cell stage in Oc- 

 tober, and that they pass the winter in this condition. In 1898 

 the same investigator 20 reported that this is true of other 

 species of Salix; that in Corylus americana (Fig. 8, B) and 

 Alnus glutinosa the midwinter catkins contain pollen ready for 

 shedding with the generative cell formed ; that in Populus 

 monilifera (Fig. 8, A) the primary sporogenous cells are 

 found in July and the mother-cell stage in October, the latter 

 condition persisting through the winter; and that in Hepatica 



