ix] THALLUS OF PODOSTEMADS 1 1 7 



esting results 1 ." Even the Tristichaceae, which do not possess 

 these polymorphic thalli, show "remarkable similarities in 

 morphological features, and in the arrangement and anatomy 

 of the leaves, to many mosses or liverworts, especially to those 

 of wet situations 1 .'* The specific and varietal names given to 

 various members of these families such as bryoides^ fucoides^ 

 selaginoides and lichenoides speak eloquently of their striking 

 resemblance to the lower plants, which the botanists who named 

 them have felt impelled to emphasize 1 . 



The genus Lawia differs from those hitherto mentioned in 

 having a thallus which is not of root nature, but which origi- 

 nates by the fusion of flattened, dorsiventral shoots, while 

 Castelnavia also has a shoot thallus. In Lawia foliosa 2 * the 

 small thallus adheres so closely to the stones that it cannot 

 be separated from them. There are no haptera, but the thallus 

 is attached by hairs. The small simple leaves are without sto- 

 mates or vascular bundles. They have a midrib of elongated 

 cells, but their structure is altogether more simple than that of 

 the leaves of many Liverworts. In Lawia xeylanica 1 the hypo- 

 cotyl, produced on the germination of the seeds, bends down 

 to the rock and becomes attached to it by unicellular rhizoids 

 from the superficial cells. The hypocotyl then expands and forms 

 a relatively large surface of attachment. 



The internal structure of the Podostemads is similar to that 

 of many other submerged plants in reduction of xylem, absence 

 of stomates, and the presence of chlorophyll in the epidermis. 

 On the other hand, a character in which these plants diverge 

 from other hydrophytes is the presence of large quantities of 

 silica in the cells 3 . It seems on the whole most probable that 

 this silica is merely a useless by-product of the plant's meta- 

 bolism 4 . It has been suggested that it serves as a protection 

 against the attacks of animals 5 , but there seems little evidence 



1 Willis,J. .(1902). 



2 Goebel, K. (i8S(f) and (1891-1893). In Goebel's earlier account 

 this plant is called Terniola (longipes?). 3 Goebel, K. (1891-1893). 



4 Matthiesen, F. (1908). 5 Wachter, W. (1897!). 



