PAST HISTORIES OF PLANT FAMILIES 165 



highly organized independent higher plants and their 

 simple parasites. The higher plants have changed pro- 

 foundly since that time, stimulated by ever-changing 

 surroundings, but the parasites living within them are 

 now much as they were then, just sufficiently highly 

 organized to rob and reproduce. 



A form of fungus inhabitant which seems to be 

 useful to the higher plant appears also to have existed 

 in Palaeozoic times, viz. Mycorhiza. In the roots of 

 many living trees, particularly such as the Beech and 

 its allies, the cells of the outer layers are penetrated by 

 many fungal forms which live in association with the 

 tree and do it 

 some service at 

 the same time as 

 gaining some- 

 thing for them- ' ' ^ 



Selves. Fjg> I20i _ Fossil Leaf / with Nests of Infesting Fungal 



CUrioUS, and aS Spores/ on its lower side 



yet incompletely 



understood physiological relation between the higher 

 plants and the fungi, existed so far back as the Palaeozoic 

 period, from which roots have been described whose 

 cells were packed with minute organisms apparently 

 identical with Mycorhiza. 



. Green Algce (pond weeds). Many impressions 

 have been described as algae from time to time, numbers 

 of which have since been shown to be a variety of other 

 things, sometimes not plants at all. Other impressions 

 may really be those of algae, but hitherto they have added 

 practically nothing to our knowledge of the group. 



Several genera of algae coat themselves with cal- 

 careous matter while they are alive, much in the same 

 way as do the Charas, and of these, as is natural, there 

 are quite a number of fossil remains from Tertiary and 

 Mesozoic rocks. This is still more the case in the group 

 of the Red Alga; (seaweeds), of which the calcareous- 



