A FACTOR IN '\:HE EVOLUTiON OF PLANTS. 



By I'rof. Horace .\ti[elstan W'ager^ A.R.C.Sc. 



Water is of the utmost importance to all protoplasm, not 

 only for strtictural ]nirposes, but for all its chief functions. At 

 the beginning of the evolutionary scale both animals and plants 

 were aquatic. Water is tal<en into all plants by the process of 

 osmosi>. As plants became more complex the process of 

 osmosis was extended to include not only absorption of water 

 from without, but also the ]:)assage of water from cell to cell. 

 It was this extension that enaljled plants to commence a terres- 

 trial nidde of existence. Complexity could only increase if 

 water conduction became improved. In this respect diiferen- 

 tiation first apjjeared in })lants in the form of a special part set 

 aside for water absorption, that is, rhizoids. roots, etc. Then 

 a special tissue for water conduction became differentiated. 

 This consisted of cells in definite longitudinal rows becoming 

 fused and forming long tube-like structiu'es called vessels. 

 These vessels were sj^ecially thickened in \arious ways, and many 

 of the cross walls disappeared. Water appears to pass up such 

 vessels, although still by osmosis from cell to cell, with more 

 readiness than in ordinar\- cells — osmosis taking place through 

 the parts of the vessel walls left tinthickened. These vessels 

 lose their protoplasm and become mechanically ftinctional for 

 water conduction. They pass right throtigh the phmt, and are 

 continually being lengthened both at the apex of the stem and 

 the apex of the root. It nnist be noted that these vessels never 

 have continuous cavities. The length between cross walls varies 

 to a great extent in different plants, but a millimetre may be 

 considered long for stich divisions. The vessels are produced 

 in the first instance at the growing points, and are continuous 

 right through the plant. In many ])lants the number of vessels 

 so produced suffices for its needs. Now. it is well-known that 

 these vessels, although produced for a definite purpose, can only 

 function for a short time as water carriers. Why should these 

 vessels, which act on a jmrely mechanical principle, so soon fall 

 out of use or even fall out of use at all, or wh} did not plants, 

 as it were, discover a more lasting kind of vessel? We must 

 take it for granted that plants did the best imaginable consider- 

 ing all the difficulties that had to be overcome. However, seeing 

 the distance that water travels in some plants, we are dealing 

 Avith the most wonderful system of water conduction in the 

 world. The explanation of the disuse of the vessels appears to 

 be thic. All water contains a certain amount of air in solution,, 

 the amount varying according to the temperature and pressure. 

 Water passing up a vessel is exposed especially to variations in 

 temperature, so that of necessity a small amount of air must 

 come iiut of solution when the tem]:)erature of the water rises. 

 This air cann(^t pass through the vessel, so that it collects in smalt 

 bubbles in the segments of the vessel, whilst the water which 

 gave it out passes on. As more water passes up, and as the 

 temperature a^ain varies, it is possible that the air is again taken 

 into solution, but the final result is that more air comes out of 



