6oo 



A TEXTBOOK OF THEORETICAL BOTANY 



the leaf tissue ; secondly the glossopodium, a rounded mass of large 

 empty-looking cells ; thirdly the ligule proper which turns upwards and 

 lies close against the leaf surface, and lastly an apex of dead cells. In the 

 tissue of the leaf around the lower part of the ligule there lies a zone of 

 short tracheids, not connected to the leaf bundle, which may be compared 

 to the transfusion tissue of the leaves of Gymnosperms (see p. 673). 



It is notable that the ligule appears very early in the development of the 

 leaf rudiment and grows much more rapidly, overtopping and curling over 

 the apex of the leaf itself. Its cells appear at this stage to be secretory, as 

 they have very large vacuoles and there is no surface cuticle. It is probable 

 that it secretes mucilage to moisten the stem apex and the young leaves. If 

 so, this function is soon outgrown, and in the mature leaf it has been left 

 so far behind in growth that it seems to be only a functionless vestige, devoid 

 of chlorophyll or starch. It does not, however, wither away, as in Selaginella. 

 The lower portion of the ligule is partially covered by an upward flap of leaf 

 tissue — the labium. Both this and the velum develop from cells immediately 

 above the young sporangium and are not sterilized sporogenous tissue as 

 has been stated. 



Development of the IMegasporangium 



The megasporangium is very large, about 5 mm. long by 3 mm. broad. 

 The cells are at first all alike, but at an early stage certain cells increase in 

 size and are recognizable as the spore mother cells, while the cells of the 

 surrounding tissue continue to divide and grow. This difl^erentiation of spore 

 mother cells in the midst of a parenchymatous tissue recalls strikingly the 

 growth of an embryo sac in the nucellus of the Seed Plants, and the resemblance 

 is increased by the enclosure of the sporangium within an integument of 

 leaf tissue. Eventually certain of the mother cells outgrow the others and 

 each divides to produce four megaspores (Fig. 611). 



-^ — sporangium wall 



Trabecula 

 Megaspore 



Fig. 611. — Isoetes lacustris. Transverse section of mega - 

 sporophyll passing through the megasporangium. 



