^l8 A FACTOR IN THE EVOLUTION OF PLANTS. 



solution than is taken back into the continuous current. In 

 this way bubbles of air must collect in the segments of the 

 vessels. It must also follow that the bubbles slowly increase 

 in size until the amount of air so imprisoned in the segments 

 of the vessel interferes with its function. The time comes when 

 the vessel is thrown completely out of action, and the vessel 

 becomes simply full of air. The length of life of a ]jlant, there- 

 fore, depends upon the length of time that its vessels can func- 

 tion as water carriers. The period of functional activity is 

 obviously difficult to arrive at, but it is usually considered to 

 vary from a few months to not much more than a year. We 

 now know, of course, that the length of life of a plant is con- 

 comitant with its ability to produce more vessels. The special 

 method by which this is carried on is well known. It is in trees 

 that longevity, together with secondary thickening, has l)ecome 

 most marked. The usual explanation of secondary thickening 

 is that it takes place in order to keep tip a supply of water to 

 the increasing plant, or that the trunk of a tree increases in size 

 in order to uj^hold the ever-increasing crown. Now, all forms 

 of life possess great latent powers which can be brought out at 

 any time. In plants this is shown by the power to survive 

 unfavourable conditions, to recover from wounds, devastation 

 by insects, etc. As vessels are permanent structures, stems that 

 l)roduce more vessels must, of course, increase in girth, and in 

 the nature of the case the new wood produced would be likely 

 to be more rather than less that already produced. The response 

 to this would be that the latent povrers in plants would allow for 

 an increase in the foliage, so that as the wood increased the 

 foliage would increase rather more in proportion. The crown 

 of a tree, therefore, is the outcome of the latent power possessed 

 by the tree, and is ])roduced more as a response to the necessity 

 of the tree to supply water-conductive tissue to rei:)lacc that 

 thrown out of action. In plants, in which more water is carried 

 up proportionately, the (juicker will the vessels be thrown out 

 of action. This is the case with most annual herbaceous plants, 

 i.e.. sunflower, dahlia, etc., so that secondary thickening is neces- 

 sary in such ])lants. l^lants that have no secondary thickening 

 are in |)ractically all cases short-lived, i.e.. monocotyledons. 

 ?^ran\- monocotvledons are perennials in so far that by storage 

 in bulbs, tubers, etc., they can send up new shoots in each 

 ensuing growing season, i.e., lilies, asparagus, etc. Any mono- 

 cotvledons wdth persistent stems, however, have a special mode 

 of secondarv thickening, i.e.. ]'itcea. Draecciia. etc. Ferns and 

 grasses, with underground stems, continuallv send out new 

 roots, whilst the stem dies away behind. Ferns, however, 

 together with mosses and liverworts, have great powers of 

 absorption of water by the leaves, and in most cases these plants 

 ran only live in damp places. The highest ty])e of plant life 

 is exemplified in a tree, and this state has been reached in evolu- 

 tion ]:»robably along the lines of water conduction.. The physical 

 propertv of water, therefore, of holding air in solution in vary- 

 in? (luantities, has in all probability played an important part 

 in the evolution of plants. 



