Lecture XVI. 125 



like ingrowths projecting into the cavity of the cell. Very little 

 protoplasm can be seen in any of these rhizoids, they are chiefly 

 filled with water and their function, in addition to fixing the plant, 

 seems to be drawing water from the soil to the plant. Of course 

 this water will be a dilute solution of salts and hence these 

 rhizoids supply the plant not only with water but also with all the 

 other constituents it requires as food, except carbon. If the de- 

 mands of the plant exceed the supply from the soil evidently the 

 walls of these flexible tubes will tend to collapse. Probably the 

 pegs on the inner walls are to guard against complete collapse 

 which would close these channels of supply. 



A curious phenomenon is often seen in these rhizoids. When 

 a rhizoid loses its protoplasm, the cells adjoining its base grow into 

 it and produce new rhizoids which replace the old one ; as many 

 as three or four of these secondary rhizoids may be found within 

 the wall of the primary one. 



From this short sketch we see that although the thallus of 

 Marchantia is not differentiated into distinct vegetative organs, 

 its cellular differentiation is considerable, so that we may recognise 

 in it a much more highly developed plant than any of the types 

 we have hitherto studied. Greater division of labour is necessi- 

 tated since the thallus is seldom completely submerged, and hence 

 while it would often be useless for the upper surface of the thallus 

 to be absorbent, it is necessary that the function of absorption 

 should be always possible by the under surface to make up for 

 evaporation and to allow of growth. Then also development of 

 the storage tissue enables the plant to tide over periods of drought, 

 but the thickening of the thallus which is necessary to secure this 

 end cuts off the lower surface from the light and necessitates the 

 limitation of the photosynthetic tissues to the upper surface. The 

 dorsal position of the thin walled photosynthetic cells necessitates 

 protection from evaporation : at the same time free access of the 

 air and its carbon dioxide is required. The chambered structure 

 of the upper tissues with pores in the roofs of these chambers 

 meets both these requirements. 



Marchantia has two prolific methods of asexual reproduction. 

 The first is a purely vegetative process and depends upon the 

 short-lived nature of the vegetative cells. These cells appear 

 normally to have a span of life of about one or two months. As 

 the new cells are formed at the tips of the branches, the cells at 

 a short distance from these tips become senescent and die. As 

 soon as the cells forming the bifurcation uniting two branches are 

 involved in this senile decay the two branches become independent 



