PPwIMAKY CAUSES OF VARIETY IN PLANT STRUCTURE. 



By George Masses. 



Living matter retains its individuality only so long as it is the seat of 

 a definite series of physical and chemical changes ; in other words, 

 cessation, loss of balance amongst the requisite forces, or the interpo- 

 lation of an unnecessary one, are the only known causes of death. 

 The harmonious working of the essential forces is always subject to 

 the effects of others antagonistic, hence the variety of' structure 

 presented by plants, which, taking the simplest forms as a starting- 

 point, illustrates the relative requirements to counteract the' preju- 

 dicial effects of conflicting agencies. Water is the medium in which 

 the greatest amount of vital activity can be accomplished with the 

 least amount of specialisation, hence the simplicity of structure com- 

 mon to aquatic plants ; not because they do less work in proportion to 

 size than other plants, but because favourable surroundings enable 

 them to do the required amount with very simple organisation : and 

 complication of structure increases in proportion as vegetable life 

 recedes from its primordial aquatic birthplace. Provision against 

 dessication is generally very manifest in plant structure, least so in 

 Algsfi, where every part of the surface is pervious to water and gaseous 

 diffusion, and the minute, solitary, unicellular species would be the first 

 to suffer from drought. The first departure from this primitive 

 simplicity in the way of protection, is illustrated by such genera as 

 Pahnella, Glceoci/stis, and Nostoc, where the individuals of several 

 generations become agglutinated together, forming colonies frequently 

 of considerable size ; or dense felt-like strata are produced by some 

 species of Spirogyra and Conferva ; in both instances each component 

 plant retains completely both its morphological and physiological 

 individuality. Such conglomerations present no approach to anything 

 like specialisation of parts or division of labour, and the explanation 

 that most naturally suggests itself is —resistance to drought — on the 

 principle that a given amount of matter in one piece takes a longer 

 time to become dry than the same amount broken up into numerous 

 pieces. The correctness of this view is supported by the fact that the 

 colony-forming species have a much wider distribution than their 

 solitary relations, due to being able to survive a wider range of 

 surrounding changes. Out of the fifty-one species belonging to the 

 PalmellacecB enumerated in Cooke's " British Fresh-water Alga3," six of 

 the rarest species belong to four genera, in which the individuals are 

 N.S., Vol. ix. Feb., 1884. 



