40 



MORPHOLOGY OF ANGIOSPERMS 



four distinct sporogenous groups are formed, each surrounded 

 by its own tapetum. As a result, the mature anther seems to 

 contain the usual four sporangia. Such a case makes the defi- 

 nition of a sporangium difficult. If a single archesporium is 

 the criterion, Lemna has a single sporangium ; if a group of 

 mother-cells invested by a tapetum is the criterion, it has four 

 sporangia. The explanation probably lies in the fact that the 

 whole outer layer of the periblem is capable of becoming trans- 



Fig. 14. — Lemna minor. Development of microsporangium and sporogrenous tissue. A 

 young stamen with sporogenous cells. £, two young stamens ; in the one at the 

 left the sporogenous tissue is becoming divided by a sterile plate. 0, a more ad- 

 vanced stage than B. D, a single stamen showing the four masses of sporogenous 

 tissue well separated by sterile plates. A > 1100; £, C, D x 712. — After Cald- 

 well. 23 



formed into an archesporium, and that while in ordinary cases 

 archesporial tissue is developed in four separate regions, in 

 Lemna the conditions favor a more general development. To 

 divide a large sporogenous mass by sterile plates for better nu- 

 trition is too common to call for any special remark. As for 

 the definition of a sporangium, it is probably not best to define 

 it too rigidly, but to use the term as one of convenience. From 





