2 40 POPULAR SCIENCE MONTHLY. 



of its weight which the air can not carry; and it so constructs certain 

 parts, for instance the leaves, of nearly all but the pines and their allies, 

 that their form is best fitted for floating. The leaves are the organs 

 in which most food is made. Their efficiency depends upon the amount 

 of light which they can absorb, and they will evidently absorb most 

 light if they are flat and placed at right angles to the rays as they come 

 from the sun. This may be the main reason for the expanded form 

 of the leaves, and it is the only reason which has been proved by 

 experiment. But it is evident that a leaf is buoyed up more strongly 

 and, therefore, requires less mechanical support if it is flat and more 

 01 less horizontal than if it were vertical or if it were cylindrical or 

 cubical. Comparing weight for weight, we find more mechanical tissue 

 in the pine-needle than in the flat leaf. And we find no such mechanical 

 tissues even in the largest and longest submersed aquatics, some of 

 which are as long as trees are tall. 



The amount of mechanically strengthening tissue in a part or a 

 plant has been proved by experiment to depend upon the amount of 

 mechanical strain to which it is exposed. Garden plants which ordi- 

 narily carry the weight of their branches will be mechanically much 

 weaker if supported on trellises. Conversely climbers and prostrate 

 plants, if subjected to mechanical pull, will develop strengthening 

 tissues which they ordinarily do not form. In these cases, the so-called 

 inherited tendency to form or not to form mechanically strengthening 

 tissues is so promptly overcome in the individuals experimented upon 

 as to suggest some doubt whether there is such a tendency at all, 

 whether the structure and behavior of living things is not more due to 

 the influence of their surroundings than to inheritance. 



We may conclude, then, that the presence in erect land plants of 

 mechanically supporting tissues which are never found in submersed 

 aquatics is not mere coincidence. The difference in the mechanical 

 tissues of these plants is due, not to the differences in their places in 

 any scheme of classification or to their degree of evolution, but to the 

 differences in the buoyancy of air and water. Aquatic plants do 

 develop mechanical tissues, but they resist the pullings, bendings and 

 blunt blows which the waves give. These tissues can not support much 

 weight. 



The strength of the submersed aquatic will vary greatly according 

 as it is a floating or an attached organism. All submersed aquatics 

 which are unattached are mechanically weak and they are usually 

 small, whereas those which are attached must develop a certain amount 

 of mechanical strength to resist the tugging of the free parts against the 

 holdfasts. Compare, for instance, Spirogyra and fresh-water Cladoph- 

 ora, plants of somewhat similar size, structure and situation. A 

 Cladophora filament will break only under a much stronger pull than 



