110 THE BIOLOGY OF A PLANT. 



function, however, is their conductivity, since they serve for 

 the transportation (especially) of the water for evaporation by 

 the leaf (transpiration), and for the carriage (through the sieve- 

 tubes) of the undissolved and indiffusible proteids. The funda- 

 mental tissues are devoted either to sharing the special duties 

 of the other systems, as in the case of the sclerotic parenchyma 

 abutting upon the epidermal tissue in the rhizome (p. 77), and the 

 sclerotic prosenchyma which appears to behave like the fibro-vas- 

 cular tissues ; or to nutritive and metabolic functions, as in the 

 mesophyll (p. 86) and the parenchyma of the rhizome. 



Reproduction. It is not known whether the brake ever dies of 

 old age. Barring accidents, growth at the apical buds seems to 

 be unlimited, keeping pace with death of the hinder parts of the 

 rhizome (p. 69). But whether the individual dies or not, ample 

 provision against the death of the race is made in the function of 

 reproduction. Although reproduction is useless to the individual 

 and even entails upon it serious annual losses of matter and 

 energy, yet to this function every part of the plant directly or 

 indirectly contributes. The reproductive germs are carefully 

 prepared ; are provided with a stock of food sufficient for the 

 earliest stages of development ; and are endowed with the pecul- 

 iar powers and limitations of Pteris aqnilina, which influence 

 their life-history at every step and are by them transmitted in 

 turn to their descendants. In short, reproduction is the supreme 

 function of the plant. If we may paraphrase the words of Michael 

 Foster, the oosphere is the goal of individual existence, and life 

 is a cycle beginning with the oosphere and continually coming 

 round to it again. 



PRACTICAL STUDY. 



A. Introductory. 



1. If possible, visit some place where Pteris aquilina grows 

 abundantly. Study it in situ, observing closely the nature of its 

 immediate environment, whether wet or dry, shady or sunny, 

 etc., and the nature of the soil. Dig up an entire plant, follow- 

 ing the ramifications of the rhizome, and observing its depth in 

 the soil, connections with the leaves, etc. 



2. Observe in a whole plant the form and size ; aerial and 

 underground parts ; axis and appendages ; rhizome and branches ; 



