INTrxODUCTION TO CllYPTOGAMIC BOTANY. 57 



increase takes place by the division of the last-formed cells of 

 what may be called the wood, as is the case in Dicotyledons, 

 we»have still something very distinct, though bearing a very 

 decided analogy. The latter mode of growth is the more 

 probable, when the similar formation in Usnea melaxantha 

 is taken into account. In this case the structure of the outer 

 portion of the medullary substance, which is often at length 

 quite free from the cortical layer, is so loose, though still capa- 

 ble of growth, and of producing, at times, gonidia, that it can 

 increase, independently of the other, and so imitate, very 

 closely, exogenous growth. 



43. In Usnea melaxantha there is, however, as stated 

 by Dr. Hooker, in the Antarctic Flora, something very dis- 

 tinct from what takes place in other Lichens, as far as is at 

 present known. " Proceeding from the circumference there 

 is, first, a horny coloured cortical layer, answering to what is 

 called the cortical layer in Lessonia, and to which, in that 

 plant also, the coloured chromule is chiefly confined : secondly, 

 the layers of intermediate lax tissue, successively deposited, 

 though much more obscurely than in Lessonia: and thirdly, 

 the central thread, which is a stout axis answering to the 

 elliptic core of Lessonia, but in this Lichen becoming so lax 

 towards the centre as to inclose a cavity in the older stems/' 

 Here, again, we have a close imitation of exogenous growth ; 

 added to which, in some Lichens, there is often a solution 

 of continuity between the cortical and medullary layers; but no 

 person would pretend that this is any sign of affinity, though 

 it is curious that the laminae in the Lessonia are analogous in 

 their effect to leaves,* and that this mode of growth should be 

 exhibited as at present known only in the noblest of all Lichens. 

 It is quite evident, however, even in species where no zones 

 exist, that increase in size takes place in the same fashion. 

 It is very curious, that in specimens of Usnea melaxantha, just 

 received from the Falkland Islands, sections of which are given 

 above (Fig. 1 5), though there is no such exogenous appearance, 



* According to Schulz, in Flora 1853, the zones in the stems of Lami- 

 naria are developed upwards, and coincident with the development of 

 additional roots. 



