INTRODUCTION TO CRYPTOGAMIC BOTANY. 9 



part, and not to their fruit ; but even then the law is far from 

 general. In such, Diatomacece for instance, as Isthmia, and 

 Aonphitetras, (Fig. 7), the development of the threads, though 

 centrifugal, is by no means external ; in ferns, the growth of 

 the stems is clearly endogenous rather than acrogenous, and 

 the same may be said of several of the more robust Algoe, 

 which seem, at first sight, to have an exogenous mode of 

 growth (Fig. 14), while those which increase by the repeated 

 division of the cells into four, increase towards the centre quite 

 as much as towards the circumference. Indeed, if Hartig* and 

 Trecul'sf views of the development of Exogens be adopted, the 

 mode of increase of the woody tissue resembles perfectly that of 

 many Conferva3, the new growth being supplied by the rej^eated 

 division of the terminal cells of the component threads of both 

 the alburnum and liber. It is not indeed pretended that other 

 modes of division do not exist, but the more normal mode of 

 growth really shows the tissue to be more closely identical 

 with Confervse than even Agardh imagined. It is very doubt- 

 ful, however, whether this learned algologist ever intended 

 more than the indication of a close analogy or, to advance a 

 step further, a distinct representation. 



6. A third distinction, and one of high importance is, that 

 though Cryptogams have, at least many of them, decided dif- 

 ferences of sex, so that they cannot correctly be termed 

 Agamous, there are no true pistils, neither are there anthers, 

 supposing the proper function of anthers to be the production of 

 pollen. The bodies which in ferns, mosses, and other of the 

 higher Cryptogams, perform the function of pollen, and which 

 are produced in a peculiar cellular tissue much after the 

 fashion of pollen grains, approach much more closely in form 

 and general appearance to spermatozoa ; and though there be 

 sometimes distinct organs for their reception, imjoregnation 

 takes place immediately by simple contact, and not by means 

 of a thread germinating from a pollen grain. Even amongst 



* Hartig Ueber die Entwickelung des Jahresriuges der Holzpflauzen, 

 Bot. Zeit. 1853, p. 553. 

 t Trecul iu Auu. d. Sc. Nat. Ser. iii., vol. 17, p. 250 ; vol. 19, p. 157, 



258. 



