EXTERNAL CONFIGURATION OF PLANTS. 189 



divisions of the frond allow free passage to the 

 wind. But undoubtedly the graceful forms of the 

 tree-ferns offer more striking examples than any to 

 be found in our climate of plants admirably adapted 

 to do battle with the storm. The divided feathery 

 fronds crowning the tall slender stem, and the 

 elegant j)alm-like habit, cannot fail to excite admir- 

 ation. If possible the internal structure is more 

 interesting still; for the vascular system of ferns, 

 forming what is called a "netted cylinder," is an 

 example of the lattice-girder principle. The vascular 

 framework of a fern reminds one of the tall signal- 

 posts of iron lattice-work so connnon on all our 

 railway lines. In ferns the vascular system is 

 reinforced by schlerenchymatous structures formed 

 in the ground tissue of the stem and stipes. The 

 minute internal structure of the fern is perhaps one 

 of the most astonishing things Avhich the microscope 

 has disclosed. 



From our present point of view both the external 

 form and internal structure of the Equisetacece are 

 highly instructive. In this order there are several 

 mechanical contrivances intended to enable the 

 plants to sustain the force of the wind. Equisetuin 

 sylvaficum looks like a miniature hr-tree, and the un- 

 branched si)ecies bear a strong resemblance to rushes 

 and grasses — plants highly specialised in this respect. 

 The surface of the stem of Equisetum is ribbed or 

 fluted. Now, while the strength of a column 

 depends on the area of its transverse section, it is 

 well known that in columns of equal section the 

 fluted column is stronger than the smooth one, 

 because each ridge acts somewhat in the same way 

 as a stay or buttress. The ridges on the stem are 

 caused by the flbro-vascular bundles, which are 

 arranged in a circle. The peculiar way in which 

 these bundles anastomose at the nodes of the stem 

 is highly advantageous, and must contribute greatly 

 to the strength of the stem. This uniting or 

 anastomosing of the bundles serves to brace together 



