30 PLANTS AND MAN 



protein synthesis does not depend upon chlorophyll or sunlight, 

 much of this soluble sugar is utilized by every cell in making new 

 protoplasm. Stem growth in diameter is the result of the activity 

 of a growth zone beneath the epidermis where cell division con- 

 tinually takes place. The growth of the blade is similarly the 

 result of cell division, often resulting in such a large surface that it 

 becomes frayed and broken by waves and ocean currents. In 

 contrast to the pond scums, the kelps have a more definite shape 

 and more limited growth. The tough, leathery outer epidermal 

 layers function well for protection, preventing dessication of the 

 underlying tissues when the kelp is by chance left stranded at low 

 tide — an experience which would be fatal to a desmid or to a 

 pond scum. 



A Primitive Thallus Land Plant 



The transition from an aquatic to a terrestrial mode of living 

 imposed severe demands upon the plant body. In all three of the 

 preceding plants the body is able to secure easily the essential 

 inorganic materials from the surrounding water; into which the 

 plant may also excrete waste substances. There is little need for 

 an impervious epidermis to prevent loss of water from the proto- 

 plasm, or for supporting tissues. Hundred-foot kelps are buoyed 

 by the water to such an extent that they need no skeletal support 

 as required by a land plant of equal size. But life on land brings 

 with it problems associated with securing water and dissolved 

 minerals, with developing protective tissues, and with production 

 of a skeleton if the plant is to attain any size. 



Liverw^orTs (fig. 11) are a group of simple land plants liv- 

 ing in the shaded and moist habitats of wooded ravines and 

 stream banks. These plants are more or less transitional in struc- 

 ture between the green thallus plants of ponds and oceans and 

 the more highly organized flowering plants. Marchantia, a typical 

 liverwort, has a flattened and ribbon-like body a few inches in 

 length which clings closely to the ground. It is a thallus plant, 

 having no diff'erentiation into roots, stems or leaves. However it 

 exhibits a certain amount of tissue diflferentiation — greater than 

 in the kelp — and is characterized by some of the modifications 

 which are necessary for land living. It has a compact multi- 



