318 



BOTANY 



the sporophyte is entirely immersed no stomata develop, but where the leaf is 

 exposed to the air, stomata are always present. Neither stem nor root develops 

 a single apical cell. 



The Root 



The arrangement of the tissues at the root-apex is not unlike that found in 

 some Spermatophytes. There may be distinguished three initials, one for the 

 plerome, one for the inner cortex, and one for the remaining outer tissues. The 

 vascular bundle is monarch. 



The Sporangium 



The derelopment of the sporangium (Fig. 285) is not unlike that of Lycopo- 

 dium. According to K. Wilson Smith, who has recently studied the develop- 



Fio. 285. Isoetes echinospora. A, section of young sporophyll (x 325) ; I, ligule; 

 the sporangial cells have the nuclei shown. B, section of a portion of a young 

 macrosporangium (x 325) ; the sporogenous cells have the nuclei indicated. C, 

 cross-section of base of sporophyll, with young microsporangium (x 25) ; v, velum ; 

 t, trabeculae. (After WILSON SMITH.) 



ment in Isoetes echinospora, the young sporangium arises from a group of 

 superficial cells, some of which, by periclinal divisions, give rise to an inner 

 layer of sporogenous tissue. All of the sporogenous cells do not develop, and 

 these sterile cell-masses form rods or strands (Trabeculse) , partially dividing the 

 mass of fertile cells. In their early stages the macro- and microsporangia are 

 not distinguishable, and it is claimed that the mother-cell of a macrospore tetrad 

 cannot be traced to the division of a definite hypodermal cell, as was formerly 

 supposed to be the case. The microspores are usually of the bilateral type, the 

 macrospores, tetrahedral. The number of microspores in L echinospora is 

 150,000 to 300,000, of macrospores 150 to 300. 



