650 



A TEXTBOOK OF THEORETICAL BOTANY 



in descent from a very early stage, and that there is a reasonable doubt whether 

 the two types of leaf are morphologically equivalent. 



FERTILE 

 TELOME 



STERILE 

 TELOME 



The Telome Theory 



The second morphological theory, which was mentioned on p. 646, 

 throws further light on the question. It is called the Telome theory and is 

 due to the work of Zimmermann. He goes back to the earliest and simplest 

 type of vascular plant known, that in the Psilophytales (see Volume III) 

 in which the body consists of branching leafless axes, which are all 

 substantially alike. In some cases these may be arranged dichotomously, 



in others monopodially, and the difference 

 may have arisen by " overtopping," as has 

 been suggested above, but in either case 

 Zimmermann regards these naked axes 

 as the primitive units of construction of 

 the cormophyte, which is thus regarded 

 as primarily a system of axes. Each of the 

 terminal branches in such a system he calls 

 a telome. Each telome is an ultimate 

 branch on an older axis or mesome. Some 

 telomes are sterile, others bear terminal 

 sporangia and are therefore fertile (Fig. 656). 

 Telomes also tend to unite into groups, 

 called syntelomes, which may be all 

 sterile, all fertile, or mixed. According to 

 this theory the earliest leaves are flattened 

 telomes or phylloids, and the sporangium 

 in all cases, together with its stalk or 

 sporangiophore, is a fertile telome. Com- 

 pound sporangiophores, like those of Eqiii- 

 setuni, are syntelomes. The phylloids are 

 the prototypes of the microphyllous foliage 

 leaves. In the most primitive condition 

 the sterile and fertile telomes were ap- 



'"''■tf^'^:S^:S^:l^^'::£iZ f^^^^^^y q"i«= independent of each other. 



plant showing a body built of telome The axillary position which the sporangial 



and mesome units, the telomes 4. i • t • j l u 



being both fertile and sterile, telomes occupy m Lycopsida has been 

 {After Zimmermann.) arrived at by secondary shifting of the 



telomes, and Fig. 657 shows three ways 

 in which the change may have taken place. Equisetum is assumed to be the 

 product of a line of evolution in which the change did not occur, and 

 consequently it still produces its sporangia independently of its leaves. 

 It must be admitted that it is difficult to accept the idea that the very 

 numerous, closely set and spirally arranged leaves of the Lycopodium type 

 have been differentiated from axial structures, and some morphologists 



TELOME 



