344 



GENERAL PRINCIPLES 



which often develop a very elaborate protective tissue. This 

 often persists long after the death of the leaf as a thick inter- 

 woven mat of tough fibres. Protected in this way the stem 

 slowly increases in thickness for several years. When the pro- 

 tective tissue finally rots away and exposes the stem the tissues 



of the latter have reached their 

 final condition and the stem 

 no longer expands in diameter. 

 Thus the stem of this type 

 soon reaches a limit in diameter 

 while the growth in height 

 may continue indefinitely, a 

 condition which is decidedly 

 inferior to that of the exogen 

 stem, in w^hich growth in thick- 

 ness keeps pace with growth 

 in height. 



730. For animals, the ques- 

 tion of growth mechanics is 

 fundamentally not as difficult 

 as for plants, because the tis- 

 sues are generally more yield- 

 ing in character. At the same 

 time the problem has appeared 

 in much greater variety and 

 Fig. 213.— Section of a conch shell has been solved in more differ- 



(Fulgur) to show that it consists of a . * - 1 



spirally wound cone. X 1/2. ^^t ways. As soon as animals 



covered themselves with a 

 protecting shell they learned the trick of making that shell 

 conical in form, so that by adding to the edge or mouth of the 

 cone it grew wider as well as longer. This type of shell is 

 found in many forms, from the protozoa to the cephalopods. 

 But a flat cone offers less protection, while a long one is 

 awkward to handle. This difficulty was also soon solved by 



