x] GENERAL OUTLOOK 157 



unicellular, motile and unattached. The first step 

 was probably that of encystment in a cell-wall. In 

 some primitive forms this was soft and gelatinous, 

 and motility was retained, as is seen in Volvox. 

 But commonly the wall formed a firm coat, and 

 non-motility naturally followed. This state proved 

 effective since it conduced to protection, and offered 

 no obstacle to self-nutrition, while it led readily 

 enough to the constitution of organisms composed 

 of more than a single cell. So long as such fixed 

 and multicellular organisms grew submerged or 

 between tide-marks, the further working out was 

 not difficult. Since mechanical stability and conduc- 

 tion of water were not pressing necessities, such 

 plants were free to attain great size, and this is 

 fully realised in the larger Algae. Spread of the 

 species was sufficiently provided for by water-carriage, 

 either in the form of motile or non-motile germs. 

 In fact a non-motile and attached habit of the whole 

 plant presents few disabilities and definite advantages 

 to plants growing in water. Hence the extent and 

 the success of the Seaweed Flora as we know it. 



But it was otherwise with the non-motile vege- 

 tation which, originating as we believe from some 

 Algal source, spread to the Land, and developed 

 into the terrestrial Flora as we see it to-day. Such 

 plants are exposed in part or even wholly to an 

 atmosphere seldom saturated with moisture, and 



