298 ESSENTIALS OF BOTANY 
of the higher green plants is probably to be found among 
the remarkable group of algee (Volvocacee) to which Volvoz, 
Pleodorina (Fig. 215), and similar colonial forms belong. 
378. Remains of the Earliest Plants 
not Preserved. — The rocks in many 
parts of the earth’s crust contain fossil 
remains of plants, often in enormous 
numbers (Sects. 308, 875). But we 
may fairly suppose that none of the 
earliest plants are thus preserved, on 
account of their soft and perishable 
nature. This must have made it diffi- 
cult for them to leave impressions in 
beds of mud or sand, or for them 
to last long enough to let limestone 
Fic. 215. Pleodorina, a or other substances gradually become 
Colonial Green Alga. deposited, instead of the material of 
Aneaveclong cousins THe plant. Since the first plants were 
swimming by means of swept out of existence ages ago, our 
cilia in the direction , . 
shown by the arrow; B, Knowledge of their form, structure, 
one of the smaller cells and relationships to later plants must 
(much magnified), show- . ' 
ing the long cilia c, the be drawn from studies of the types 
eye-spot ¢ (supposed to which we know, either in the shape 
be sensitive to light), the a F 
nucleus n, the pyrenoid Of fossils or as living species. 
p, and the cup-shaped = 379, Evidence from the Life His- 
chloroplast ci. 5 wt ese 
tories of Plants. — Every individual 
seed-plant and every one of the higher spore-plants during 
its life history goes through a series of changes, — from the 
spore with which it begins to the most highly developed 
form of which that plant is capable. This gradual unfold- 
ing of organs, from a very simple spore as the starting 
point, means everything to the botanist. For in botany, 
