DIFFERENTIATION OF ORGANS IN SPERMAPHYTA 17 



of the Labiatae a 'gland-hair' possessing the same structure and the same 

 peculiar features as one growing from the epidermis I would certainly 

 call it a ' gland-hair.' The position and origin of an organ is in my view 

 one point of importance, but it is not the only criterion by which to judge. 

 Every single organ is built up through a series of regular succeeding 

 developmental stages which are based on its material nature and which 

 can be changed in certain ways. Why should the change not produce 

 its effect at the place of origin of the organ ? The antheridia of most 

 liverworts arise from surface-cells, but those of Anthoceros occur in 

 a closed depression. Are we then to regard the antheridia of Anthoceros 

 as not homologous with those of the other liverworts ? The question 

 we have to answer here is only how have the antheridia of Anthoceros 

 got into the pit, and has this deviation any special significance for them ? 

 The point is not does their position make them different from the other 

 antheridia of the liverworts ? In speaking of the organs of vegetation 

 there is no doubt that the use of the expression ' trichome,' based upon 

 the seat of origin of an organ, is in many cases very convenient, but it 

 is absurd to call many reproductive organs of plants ' trichomes,' as is 

 so often still done, simply because they arise upon the epidermis. The 

 reasons against it which I have previously established have in great 

 measure been overlooked, and I will therefore repeat them here. 



Origin of an organ from the epidermis is one aspect, but only one, 

 and that a purely technical one, of the development. What we have to 

 learn is not only how au organ arises, but before everything else, what it is, 

 and if we venture upon the statement that all ' trichomes ' arise from the 

 epidermis the converse is very apt to be assumed that everything that 

 arises out of the epidermis is a trichome. Leaf-structures also sometimes 

 arise from the outermost cell-layer of the vegetative point, as Stras- 

 burger has shown to be the case with the perianth of Ephedra, and the 

 adventitious shoots which develop upon detached leaves of Begonia arise 

 commonly, according to Hansen, from a single epidermal cell. No intelli- 

 gent man would on this account call them trichomes. There is as 

 little sense, to my thinking, in calling the sporangia of ferns ' trichomes ' 

 as is so often done, because neither in ontogeny nor in phylogeny is there 

 any evidence that a sporangium has been derived from a hair through 

 a change of its function. Although we cannot trace the phylogenetic 

 development of the Pteridophyta we know that the sporiferous generation 

 is the homologue of the sporogonium of a moss ; but by what process of 

 development the sporangia have been differentiated from a sporogonium- 

 like structure we can only conjecture. If we had a record of this history 

 we should know the ' morphological value ' of the sporangium ; for us 

 nowadays to refer it to a hair-structure is simple nonsense; in many cases the 

 hairs had probably an origin much subsequent to that of the sporangium. 



r.OEHEr, C 



