430 



PALEOBOTANY. 



a. Endogens. — Ex. Grasses, Palms, Lilies. These have endogenous 

 stems, showing no rings of growth, and the young plant possesses but a 

 single seed-lobe or " cotyledon." Hence they are often called Monocoty- 

 ledons. 



h. Exogens. — Ex. Pines and Cycads, with most ordinary shrubs, trees, 

 and flowering plants. Tlie Pines and Cycads, with the fossil Sigillaiice, 

 have the seed naked, and are hence called Gymnosperms (Gr. gumnos, 

 naked ; sperma, seed). Ordinary trees and shrubs, on the other hand, 

 have the seed protected by a seed-vessel, and are therefore called Angio- 

 s^perms. Both the Gymnosperms and Angiosperms have an exogenous mode 

 of growth, with a true bark and annual rings of growth. The seed also 

 possesses two seed-lobes or " cotyledons ; " and they are therefore often 

 spoken of as Dicotyledons. 



As regards the distribution of the principal groups of the 

 vegetable kingdom in time, the Cryptogams are, on the whole, 

 older than the Phanerogams, though the former are in many 

 instances little adapted for preservation in sedimentary 

 accumulations, and very little is known about the past oc- 

 currence of any but the more highly organised sections of 

 tlowerless plants. Taking the Algce first, we have but very 

 fragmentary evidence as to the past existence of the micro- 

 scopic plants which are known as Diatoms and Desmids. 

 The former of these, being furnished with a siliceous epi- 

 dermis, are quite capable of 

 preservation in the fossil state, 

 but tliey have, nevertheless, not 

 been as yet detected in any of 

 the older strata of the earth's 

 crust. Eecently, Count Castra- 

 cane announced the discovery 

 of Diatoms in the ashes of 

 coal ; but the investigations 

 of Professor Williamson have 

 thrown discredit upon these ob- 

 servations, and we must there- 

 fore assume that Diatoms have 

 not hitherto been detected in 

 the Carboniferous rocks. In the Tertiary deposits, however, 

 we meet with great deposits of so-called " Infusorial Earth," 

 whicli are really in great part made up of the siliceous 



Fi>,'. 692. — Siliceous envelopes of 

 Diatoms, from the " Richmond Earth." 

 Greatly magnified. 



