PROTECTION. IS 



germinating in a subdued, equally diffused light, or in 

 darkness, the spores always develop rhizoids and apical 

 growing points at opposite poles, but these points of growth 

 bear no definite relation to surroundings as indicated by 

 direction ; whereas when germinating under new conditions, 

 as when exposed to a strong side light, it may be presumed 

 that the internal causes continue to determine polarity to 

 the extent of rhizoids and apical-cell appearing at opposite 

 poles, and that the new factor, light, determines the orienta- 

 tion or direction, causing all the rhizoid poles to point 

 away from it. Finally the manifestation of spontaneous 

 movements in plants is almost entirely dependent on light 

 and temperature. This does not necessarily mean that the 

 proper combination of temperature and light is the funda- 

 mental cause of such movements ; but one thing appears to 

 be certain : admitting the existence of some additional 

 internal factor, this hypothetical factor is regulated in its 

 working by external agents. 



Protective organs in the broader sense are characteristic 

 of life, and adaptation to circumstances depends, not entirely, 

 but to a considerable extent on the power of certain 

 organisms to become so modified in their structure and 

 functions as to overcome conditions that would prove fatal 

 to the unmodified organism. As an illustration of the 

 above on a broad scale we may briefly trace the gradual 

 evolution of land vegetation from primitive aquatic forms. 

 Water is the medium in which the greatest amount of 

 vital activity can be manifested with the least amount of 

 specialization and expenditure of energy, hence the sim- 

 plicity of structure characteristic of aquatic plants. In the 

 case of land plants, provision against dessication or the 

 loss of an undue proportion of water from those parts 



