Cdvers : Some Pomts in the Biology of Hepaticce. 211 



mucilag'inou.s, so that on losing- water they become shrunk, and 

 on being- ag-ain moistened return to their orig-inal condition. 

 Mattirolo found that when plants of Grimaldia were cultivated 

 in a moist chamber, they ultimately lost the property of resisting- 

 desiccation, the ventral scales formed on the new branches being- 

 smaller and remaining- g-reen. Exactly similar results have been 

 observed by the present writer in the case of Reboulia. The 

 chief biolog^ical features of the xerophytic Marchantiales may be 

 thus summarised : (i) the sharply-marked epidermis, the cells of 

 which contain little or no chlorophyll and have thickened walls ; 

 (2) the relatively narrow and deep air-chambers, with small 

 pores ; (3) the well- 

 developed compact 

 ventral tissue, 

 adapted for the 

 storage of water 

 and reserve food- 

 materials and for 

 causing the rolling- 

 up of the thallus ; 

 (4) the larg-e and 

 deeply-col o u r ed 

 ventral scales, 

 adapted for holding 

 water by capillarity 

 and for protecting 

 the growing-point, 

 in addition to cover- 

 ing the rolled - up 

 thallus; (5) the 

 well-developed 

 tuberculate root- 

 hairs. In Preissia, 

 as noted by Gcebel, 

 the lowest ring of 

 cells surrounding 

 the ' barrel-shaped ' 

 pore is capable of 

 being narrowed so 

 as to close the pore, 

 these cells acting in 

 the same manner as 

 the ' ofuard-cells ' in 



Fig. g. — Preissia cotnmutata. I., Shows a single air- 

 chamber in vertical section, with its 'barrel-shaped' pore 

 and branched green filaments ; II., part of a longitudinal 

 section through the midrib, with portions of two fibres ; 

 III., IV., a pore in surface view (III., showing the upper 

 ring of cells ; IV., the lowest ring); V., an isolated 

 sclerotic fibre. All x about loo. 



1Q0-; Ji 



