USEFUL ALGAE — CHASE 405 



formed by the lateral cohesion of a multitude of roundish polygonal 

 (due to mutual pressure) cells that originate by the quadripartition 

 of older cells (longitudinal as well as transverse division) so causing 

 the cell growth to proceed nearly equally in all directions. In Ulva^ 

 or sea lettuce, we find the earliest type of an expanded leaf. 



Similarly, the earliest type of a stem may be traced back to the 

 cylindrical cells of the lower algae. In Conferva, whose body con- 

 sists of a number of cylindrical cells fastened end to end, all con- 

 tinually originating by the continual transverse division of an original 

 cylindrical cell, the frond or plant body continually lengthens but 

 does not make any lateral growth. It consists of a series of joints 

 and interspaces and correctly symbolizes the stem of a higher plant 

 formed of a succession of nodes and internodes. In other genera, 

 these confervoid threads branch and the branches originate at the 

 joints or nodes like the leaves and branches of the higher compound 

 plants. In still other genera of algae, the stems become flattened 

 at their simimits until leaflike parts are formed which again by the 

 loss of their lateral membranes and by the acquisition of thicker 

 midribs change back into stems. Among the most highly organized 

 algae there are leaflike lateral branches that assume the form and, 

 to an extent, the arrangement of the leaves of higher plants. But 

 even when the leaflike bodies appear most highly developed in the 

 algae, they are merely expanded branches as may be seen by observa- 

 tion of the gradual changes that take place in a young Sargassum 

 seaweed as the frond lengthens. 



The algal cells imbibe their food equally through all parts of their 

 surface and the food is passed from cell to cell toward the cells that 

 are assimilating more actively or growing more rapidly. The salts 

 and gases that compose the food of algae are dissolved in the salt 

 water or fresh water that surrounds the algal cells or they are in the 

 air or dissolved in the water in the soil about the algae. For this 

 reason the alga does not need a true root such as is found in the 

 higher plants. Wliere a rootlike organ exists in algae, as in the 

 larger seaweeds, it is a mere holdfast with the purpose of anchoring 

 the seaweed to a stone or wooden base and thus preventing the seaweed 

 from being driven about by the action of the waves. Ordinarily, in 

 the smaller kinds of seaweeds, it is a simple disk or conical expansion 

 of the base of the stem that is strongly fastened to the substance on 

 which the seaweed is growing. In the gigantic oarweeds, or Lami- 

 naria, where the frond has attained a large size and offers a propor- 

 tionate resistance to the turbulent waves of the ocean, the central disk 

 is strengthened by lateral holdfasts or disks formed at the bases 

 of side rootlike parts emitted from the lower part of the stem, just 



