298 



ESSENTIALS OP BOTANY 



of the higher green plants is probably to be found among 

 the remarkable group of algse ( Vblvocacece) to which Volvox, 

 J'leodorina (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. 303, 375). 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 



^ „, ^ _, , . or other substances gradually become 



Fig. 215. Pleodonna, a . . b j 



Colonial Green Alga, deposited, instead of the material of 



^, a cell-colony (magnified) t^e plant. Since the first plants were 



swimming by means of swept out of existence ages ago, our 



cilia in the direction , ,, f n ■ r- , , 



shown by the arrow; 5, knowledge of their form, structure, 

 one of the smaller cells and relationships to later plants must 



(much magnified) , show- , ., . , . t , 



ing the long cilia e, the "e drawn from studies of the types 

 eye-spot e (supposed to -which we know, either in the shape 



be sensitive to light) , the j. j. -i ,. . . 



nucleus n, the pyrenoid Ot fOSSllS Or aS living Species. 



p, and the cup -shaped 379.. Evidence from the Life His- 



chloroplast d. 



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, 



