[Vol. 2 



294 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



certainly seems impossible that this can be a general factor. 

 Its local effect, however, may be very considerable. 



Light varies from the equator, where it is most intense, to 

 the poles where it is least. It very decidedly limits the dis- 

 tribution as to depth. Marine algae of the benthos need light 

 and are, therefore, limited to the neritic portion of the photic 

 zone as to their general distribution. Outside of this general 

 limitation, however, it does not appear that the varying in- 

 tensity of light can be considered as a prime factor in limiting 

 floral regions and floral provinces, i.e., not alone. 



Varying temperature, however, does act directly upon algae 

 to limit their distribution, both locally and generally. It can 

 easily be recognized to be the one most important factor in 

 controlling the distribution of benthos over wide areas as 

 well as, at times, in smaller districts or spots. We recognize 

 that, in general, the species of the frigid zones, of the tem- 

 perate zones, and of the tropical zones are sufficiently different 

 to give an entirely different facies to each. Yet, in consider- 

 ing general regions, we find that they are not marked out by 

 the same parallels as are used to mark these zones geographic- 

 ally. These geographical zones, however, are established 

 more particularly as regards direction of the sun's rays and 

 the temperature of the air rather than that of the water. 



The waters concerned with the life and persistence of the 

 algae, even of the benthos, are, relatively speaking, the sur- 

 face waters, since algae seldom grow lower than at a depth 

 of 100 meters and for the most part cease at 20-30 (or at 

 times 40) meters. The normal decrease in temperature at 

 such depths is slight even in temperate waters, although, at 

 times, sufficient to account for special sporadic anomalous 

 distribution. The range in temperature under which algae, 

 in general, may carry on their full course of vegetative and 

 growth activities is from — 2°C. up to the neighborhood of 



90° C, but that for marine algae is only from — 2°C. up to 

 30 C C. (or possibly 32°C), this being the extent of ranges 

 for all surface waters of the ocean. 



A comparison between charts in which the isotherms for 

 surface temperature of the water of the oceans are laid off 



