TISSUES AND SIMPLE ORGANS, 

 y. Protection of the Leaf -tip. 



119 



The following is immediately associated with what has just 

 been stated (3). An interesting and at the same time easily 

 demonstrable means for the protection of growing organs is to be 

 found in leaves of ferns. The leaves of the ferns are an exception 



FIG. 66 b. (Diagramatic.) 



to those of numberless other plants in their mode of development 

 (Wedel). Most leaves (phanerogams) develop so that the tip or apex 

 ceases to grow first, the base last. In this and in the overlapping 

 of the leaves in the bud lies the protection for the meristematic 

 portions of these organs. Fern-leaves, however, are peculiar in 

 having a long-continued apical growth from an apical cell : the tip 

 therefore requires some form of protection for a considerable period 

 of time ; this is supplied by the well-known spiral (circinate) rolling 

 of the leaf -tips. 



(b) Protection for Areas of Intercalary Growth. 



In this category of protective adaptations we are in part con- 

 cerned with evident mechanical relations. Accordingly at least a 

 part of this discussion might have found a place in the treatment 

 of the mechanical tissue-system. I believe, however, that it may 

 well be introduced at this point. In the first instance we are con- 

 cerned with firm organs, stems in particular, which become elon- 

 gated at interpolated zones (" intercalation "). It is evident that 

 these growing zones consist of delicate and yielding tissues ; there- 



