152 THE BIOLOGY OF A PLANT. 



Physiology of the Tissue- Systems. The epidermal tissues 

 serve as the sole medium of exchange between the inner parts of 

 the plant and the environment ; they are also protective, and in 

 certain regions are useful for support. The function of repro- 

 duction also falls upon these tissues, as is shown by the develop- 

 ment of the sporangia, antheridia, and archegonia. 



The fibro-vascular tissues serve in part as a supporting 

 skeleton, for which function their richness in prosenehyma 

 and their firm continuity admirably adapt them. An equally 

 important function, however, is their conductivity, since they 

 serve for the transportation of the water for evaporation by the 

 leaf (transpiration}, and for the movement (through the sieve- 

 tubes) of the undissolved and indiffusible proteids. The/'"/"/"- 

 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. 119), and 

 the sclerotic prosenchyma which appears to behave like the libm- 

 vascular tissues; or to nutritive and metabolic functions, as in 

 the mesophyll (p. 126) and the parenchyma of the rhizome. 



The Physiology of 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. 111). But whether the indi- 

 vidual dies or not, ample provision against the death of the race 

 is made in the act of reproduction. Although reproduction ap- 

 pears to be 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 peculiar powers and limitations of 

 Pteris aquilina, which influence their life-history at every step 

 and are by them transmitted in turn to their descendants. They 

 are living portions of the parent detached for reproductive pur- 

 poses; they contain a share of protoplasm directly descended 

 from the original protoplasm of the spore from which the parent 

 came ; and thus they serve to effect that ' ' continuity of the 

 germ-plasm" to which we have already referred in dealing 

 with the earthworm. In short, reproduction is the supreme 



