2] THE VARIOUS GROUPS OF PLANTS 49 



whole tribes of northern peoples depend for the wherewithal of life. 

 Their plentiful storage of a starch-like carbohydrate also makes some 

 of them, such as ' Iceland-moss ', useful for human food, though 

 most are highly unpalatable. The use of certain Lichens as sources 

 of attractive dyes has greatly diminished with the chemical advances 

 of recent years, but some dyes, such as litmus, are still widely 

 obtained from them. 



Bryophyta 



Hepaticae : These, the Liverworts, are a smallish class of usually 

 green photosynthetic plants of relatively small size, though always 

 multicellular and visible to the unaided eye. There are two main 

 forms : the thalloid, having a thin and more or less flat, prostrate 

 plant body which tends to branch frequently and equally, and the 

 ' leafy ', consisting of a creeping central axis up to a few inches 

 long, provided with delicate leaf-like expansions. These last are 

 only one cell thick and lack a midrib ; they are usually arranged in 

 two rows, lying on either side of the often prostrate axis, with 

 commonly a third row of smaller lobes lying along the under surface. 

 Noticeable on the lower surface, especially of the thalloid types, 

 are numerous thin root-like ' rhizoids ', primarily serving the pur- 

 pose of attachment to the ground or other material on which the 

 plant grows. Often there are air-chambers or other special features 

 on the upper surface. The main photosynthesizing plants are the 

 gametophytes, so termed because they produce the gametes ; they 

 comprise the gametophvtic generation which alternates regularly 

 with the spore-producing (sporophytic) one to complete the life-cycle. 



The gametes are formed in minute male and female organs, the 

 male spermatozoids swimming freely to fertilize the passive and 

 well-protected female ' eggs ', from which, after fertilization, the 

 sporophytes develop. In most tvpes these last consist of an absorb- 

 ing foot, a more or less elongated stalk, and a roundish capsule 

 (sporangium) in w^hich the microscopic spores are produced in 

 considerable numbers. The foot is embedded in the tissue of the 

 gametophyte, from which it absorbs nourishment. This is passed 

 on to the rest of the sporophyte, w^hich in most types lacks chlorophyll 

 and is thus parasitic on the gametophyte. In some simple forms 

 there is no foot or stalk, the sporangium being embedded in the 

 gametophyte, and in one group the sporophyte is photosynthetic 

 and grows continuously from near the base. But in any case there 



