56 



INTRODUCTION TO CRYPTOGAMIC BOTANY. 



never sanction any notion of a relation to Endogens. The 

 exigencies of climbing plants, which suffer great compression, 

 require a peculiar arrangement and structure in their com- 

 ponent parts. But other instances may be produced equally 

 full of instruction. 



42. Take, as a third instance, the pseudo-exogenous growth 

 of the larger Seaweeds and Usneoid Lichens. A transverse 

 section in many of the former presents zones, formed, period by 

 period, corresponding with the development of the laminae, 

 roots, or l)ranches, as is verv visible in the stems of Lessonia. 



Fig. 15. 



a. Sections of Us)iea melaxantha. 



b. Section of stem oi Lessonia, showuig the pseudo-exogenous growth. 

 Both from specimens communicated by Dr. Hooker. 



There is no line of separation between the cortical cells and 

 the zoned portion, as though there were at once a centrifugal 

 and centripetal growth, nor are there any wedges of medul- 

 lary tissue from the central pith, which is so conspicuous. The 

 difference, then, between this and real exogenous growth is very 

 great, and whether the innermost cells of the cortical layer 

 are gradually changed, the terminal cell dividing, or whether 



