SELAGINELLA 149 



2. Coraparp tlio cell stnicture of very young leaves with that of older 

 ones. The cells, in the younger leaves, contain a single large chro- 

 raatophore, which becomes divided into a cliain of segments in the 

 older cells. 



F. The germination of spores, and the gametophytes. The microspore pro- 

 duces a very small male gametophyte, which remains contained within 

 the ruptured spore wall and develops two-ciliate sperms (see Prin- 

 ciples, Fig. 290, A, B). The megaspore gives rise to a small female 

 yametophyte, which emerges somewhat from the ruptured spore and 

 develops several archegonia (see Principles, Fig, 290, C). The develop- 

 ment of these gametophytes requires several weeks, and their study 

 demands microtome sections, which are difficult to prepare, so that 

 they can hardly be treated in an elementary course. Marsilia (Sec. 132) 

 is a better type for the study of reduced male and female gametophytes 

 associated with microspores and megaspores. 



G. The development of the sporophyte. This study, like that of gameto- 

 phytes, requires microtome sections of material difficult to obtain and to 

 cut, and is hardly practicable for a general course. Marsilia (Sec. 132) is 

 a better type to illustrate the relation of the embryo sporophyte to the 

 female gametophyte in heterosporous pteridophytes. The embi-yo de- 

 velops in the interior of the gametophyte at the end of a structure 

 called the suspensor. Three regions are differentiated, — the stem with 

 young leaves forming a bud, the root, and the large /ooi (see Principles, 

 Fig. 290, C). The foot absorbs food from the tissue of the gameto- 

 phyte within the megaspore. In certain species of Selaginella, as S. ru- 

 pestris, the sporophytes are developed while the megaspores are still held 

 mechanically by the sporophylls on the cone (see Principles, Fig. 290, 

 D). This retention of the megaspores on the sporophyte is suggestive 

 of the seed habit (see Principles, pp. 335 and 330). 



Reference. Campbell, 23. 



Questions. What are the growth habits of the type of SeUiyl- 

 nella which has been studied ? Are there any xerophytic 

 adaptations ? Describe any peculiarities in the arrangement 

 of the leaves and suggest reasons therefor. What is the 

 form and structure (morphology) of the cone ? Why are its 

 leaves called sporophylls ? What is the relation between the 

 large size of the megaspores and their production relatively 

 few in a megasporangium ? What is heterospory ? What 

 are its advantages in giving the sporophyte a better start in 



