BRYOPHYTES 



71 



54). The ventral surface puts out hair-like processes that 

 grip the substratum (Figs. 53 and 54). These hairs are 

 called rhizoids, which means " root-like," but they are not 

 like roots. They have not the structure of roots, and they 

 do not perform the work of roots except as they anchor the 

 body. 



This differentiation of the body, compelled by its position, 

 outlines regions of work that become more definite in higher 

 plants. Water enters the 

 body through the lower epi- 

 dermis, is conducted through 

 the polorless cells on the ven- 

 tral side, is brought to the 

 green cells on the dorsal side, 

 and is partly used there in 

 food manufacture. It must 

 not be supposed that all of 

 the water that enters a plant 

 is used in photosynthesis, for 

 the bulk of it keeps the pro- 

 toplasts of all the cells in a 

 condition for working, and 

 the protoplasts are losing it 

 all the time to the air. The 

 problem of the plant is to 

 see to it that the protoplasts 

 do not lose water faster than it is supplied. The picture 

 in one's mind, therefore, should be that of a stream of 

 water moving continuously through the body of the plant, 

 primarily to keep it in working condition, and incidentally to 

 supply a little for photosynthesis. 



The epidermis introduced a new problem which the Liver- 

 worts and all the higher green plants have solved. In order 

 to do their peculiar work, the green cells must be exposed to 

 the air, from which carbon dioxide is obtained, and to which 



FIG. 54. -Marchantia: a plant bearing 

 on its dorsal surface three long-stalked 

 disks with finger-like lobes and contain- 

 ing archegonia, and also two small cups 

 containing gemmae ; the archegonia are 

 found on the ventral side of the disk, 

 hanging from the angles between the 

 lobes. 



