THE MORPHOLOGICAL COMPOSITION OF PLANTS. 57 



mutually injured. Such of them, however, as happen, by 

 variations in mode of growth, to rise higher than others, 

 are more likely to flourish and leave offspring than others. 

 That is to say, natural selection will favour the more upright- 

 growing forms. Individuals with structures which lift them 

 above the rest, are the fittest for the conditions; and by the 

 continual survival of the fittest, such structures must become 

 established. There are two essentially-different ways in 

 which the integrated series of fronds above described, may be 

 modified so as to acquire the stiffness needful for maintaining 

 perpendicularity. We will consider them separately. 



A thin layer of substance gains greatly in power of resist- 

 ing a transverse strain, if it is bent round so as to form a 

 tube : witness the difference between the pliability of a sheet 

 of paper when outspread, and the rigidity of the same sheet 

 of paper when rolled up. Engineers constantly recognize 

 this truth, in devising appliances by which the greatest 

 strength shall be obtained at the smallest cost of material; 

 and among organisms, we see that natural selection habit- 

 ually establishes structures conforming to the same principle, 

 wherever lightness and stiffness are to be combined. The 

 cylindrical bones of mammals and birds, and the hollow 

 shafts of feathers, are examples. The lower plants, too, fur- 

 nish cases where the strength ' 

 needful for maintaining an 

 upright position, is acquired 

 by this rolling up of a flat 

 thallus or frond. In Fig. 77 

 we have an Alga which ap- 

 proaches towards a tubular 

 distribution of substance ; 

 and which has a consequent 

 rigidity. Sundry common 

 forms of lichen, having the 

 thallus folded into a branched tube, still more decidedly dis- 

 play the connexion between this structural arrangement 



77 



