144 MORPHOLOGY 
quence of this abandonment of leaves as foliage, photosynthesis is 
entirely a function of the green tissue of the stem. Many of the 
older forms were leafy, but all Equisetales are characterized by the 
I 
FlGS. 332-338. Equisetum arvense: 332, fertile branches from the dorsiventral 
Stem, each bearing a terminal strobilus, one of them mature; 333, sterile branch; 
334> 335. two views of a sporangiophore (so-called "sporophylJ "); 336-338, spores, 
showing the unwinding of the periniuni. After WOSSIDLO. 
