i6 PLANT LIFE. 



growing in the air is an absolute necessity. It is generally 

 admitted that the entire land vegetation originated from 

 sea-weeds, and by various phases of evolution attained to 

 the great variety of form and structure existing at the 

 present day ; and during the early attempts of seaweeds, the 

 pioneers of the vegetable kingdom, to gain a footing on dry 

 land, they were confronted with the unforeseen difficulty 

 of counteracting the grave danger resulting from dessication. 

 Primitive attempts to overcome this difficulty are seen in 

 such microscopic aquatic forms as Palmella, Glceocystis, and 



Fig. 2. — Pandorina, trwrum (x 400). A, a motile colony (or coenobium). 

 The individual cells have their cilia protrading through the gelatinous 

 envelope of the colony. B, two zoogonidia, formed by the division of 

 the cells oiA, in process of conjugation. (From Prantl.) 



Nosioc, where numerous individuals become agglutinated 

 together by a secreted substance very retentive of water, 

 thus forming colonies frequently of considerable size ; other 

 kinds form dense felt-like masses, as Spirogyra and Con- 

 ferva. Such collections of individuals present no approach 

 to anything like specialization of parts or division of labour, 



