684 



GA RDENERS' MAGAZINE. 



October 22, 1898. 



Homology of Plants. 



By Professor F. O. Bower, D.Sc, F.R.S 



( Continued from page 668. ) 



HI —Abnormalities. 



The following considerations influence me in forming an opinion as to 

 the real place of apospory and apogamy in the history of the alternating 

 generations : — 



I. The Bryophytes show remarkable uniformity of alternation ; irre- 

 gularities are few ; apogamy is not recorded ; apospory appears rarely, 

 as a physiological refuge for the destitute plant. This uniformity goes 

 along with the protected and dependent condition of the sporophyte. All 

 Pteridophytes have their embryos protected while young, and this seems 

 to have been their primitive condition. The true lesson of the Bryophites, 

 which include the simplest living Archegoniates, seems thus to be that 

 uniformity of alternation goes with a simple structure, and a protected or 

 dependent condition of the sporophyte ; and this we have reason to 

 believe was the condition of the simpler Archegoniate fruits. 



II. The distiibution of apogamy and apospory among Archegoniates 

 at large is very irregular ; the Leptosporangiate Ferns are the head- 

 quarters; but they are a pecularly specialised phylum, with free sporo- 

 phyte, exposed when mature, though protected while young. They are 

 adapted to special conditions and show a greater plasticity of development 

 than any other Pteridophytes. The F ems are subject to other abnor- 



malities than apospory and apogamy. 

 into a shoot, or the apex of the leaf into a bud. 



The root may develop directly 



I think it has been too 



readily held that the Ferns occupy a special place as a key to the morpho- 

 logical problem. We shculd bear in mind how really isolated they are ; 

 they are essentially an extreme, even an extravagant type ; they show the 

 largest sporophylls in the whole vegetable kingdom, with the largest 

 numerical output of spores from each. Many are specialised in accord- 

 ance with extreme conditions of shade and moisture. These considera- 

 tions should temper our. view of them, not only as material for normal 

 comparison, but also as exponents of abnormality. 



111. The fact that in cases of induced apogamy in Ferns archegonia 

 are first produced, clearly shows that in these cases the first intention of 

 the plant is towards a normal production of embryos, while apogamy 

 takes its ph 



as a substitutionary growth. It may remain an open 

 question how far direct apogamy will bear a similar interpretation. 



IV. The character of the aposporcus and apogamous growths is very 

 anomalous ; their position is not definite. Aposporous growths may 

 arise from the sorus and sporangia, or from the most varied points on 

 the margin or surface of the leaf. With regard to apogamy in Ferns, it 

 appears, as the result of a large number of observations, that though 

 mere is an average normal of position, still any part of the sporophyte — 

 stem, leaf, ramentum, root, sporangium, or even tracheid — may arise, 

 independently of others, from the piothallus. Single sporangia, or 

 groups, of them, may appear without vegetative organs of the sporo- 

 phyte ; leaves without other parts ; in one case, I believe, as many as ten 

 roots have been seen without any other members of the sporophyte ! 

 The close similarity of the parts thus irregularly placed to those formed 

 in regular sequence in the normal plant should be a warning of their 

 abnormality. I cannot see in them any suggestion of a primitive state. 

 Dr. Lang tells me that these exceptional developments form only a small 

 proportion of the individuals in any one culture ; still they are there, and 

 those who hold that apogamous developments are a suitable basis for 

 morphological argument must not pick and choose those cases which 

 suit their views, but must take even the most extravagant into careful 

 estimation. My own view is that these anomalous growths are not a safe 

 guide to past history. But looked u;:on as the result of a recently- 

 acquired transition from one generation already established to the other, 

 following nuclear changes, in the one case of reduction after insufficient 

 nutrition, in the other of doubling of the chromosomes following on 

 plethora apospory and apogamy are at least intelligible. We shall under- 

 stand how the transition may take place at one point or at many, while 

 the irregularity of the parts produced offers no morphological difficulty ; 

 it is rather what might have been anticipated if the transition were a ready 

 consequence of the conditions we have noted. 



Lastly, a word on Dr. Scott's utilitarian argument. He remarks 44 a 

 mode of growth which affords a perfectly efficient means of abundant 

 propagation cannot I think be dismissed as merely teratological." We 

 must be clear that utility is no certain evidence of antiquity. As refuges 

 for the physiologically destitute, apogamy and apospory may play an 

 important part now, and in so far are not to be dismissed as mere freaks 

 of nature. But in my view they would rank, as regards utility pure and 

 simple, with the formation of adventitious buds on the root-system of a 

 poplar ihat has been felled ; or with the bulbils which replace the flowers 



has been to set aside some of the difficulties which have been suggested 



in opposition to an antithetic view, and to show that the latter then™ , 'n 

 edequately cover the facts. cu ™ WIU 



Returning now to our general inquiry on homology, we see that on the 

 antithetic view the two generations are not homogenetic ; but thev mavh 

 in a high degree homoplastic, and this homoplasy may be impressed unon 

 the two generations, even in the same species, as in some Lycopods I 

 have never felt the cogency of the fact that thegametophyte of / cernuum 

 is somewhat similar in outline to the young sporophyte. Both generations 

 are exposed to similar circumstances, and may be reasonably expected to 

 have reacted alike. Moreover, the similarity of form of the u leaves" of 

 protbalJus and plant is but slight, and is not maintained in allied species 

 Their arrangement is variable. Between them also lies the essential 

 structural difference, so widespread among Archegoniate plants, that in 

 the sporophyte stomata and intercellular spaces are present, in the game- 

 tophyte they are absent. These are just such differences as point to 

 homoplastic development Most commonly, however, the homoplastic 

 development is only seen in distinct organisms, and in this sense we shall 

 rank the leaf of the Moss as the homoplast, but not the homogene, of the 

 leaf of a Lycopod or of a Fern. 



Theory of the Strobilus. 



Some years ago I submitted a theory of the strobilus in Arche- 

 goniate plants. Comparisons were drawn between Pteridophytes 

 and Bryophytes, and it was suggested that the origin of the strobilus of 

 the former was " from a body of the nature of a sporogonial head." I 

 specially pointed out at the time that my object was not a mere hunt 

 after homologies, but to obtain some reasonable view of the methods oj 

 advance in Archegoniate plants. I wish to lay special stress upon this, 

 for some appear to think that by denying an homology which I have not 

 been at pains to maintain, they invalidate this search after the methods 

 of advance. The Bryophytes as we now see them are our best guides in 

 the search after these methods, even though they may not have been in 

 the direct line of descent of Vascular Plants. As regards the comparison 

 of the strobilus with a sporogonial head, I wish to make it clear that a 

 Moss sporogonium is not specially indicated. The expression used has 

 been "the origin of the strobilus from a body of the nature of a sporo- 



-that is simply a part of the sporophyte which bears 

 spores internally as distinct from a lower vegetative region. We see in 

 more than one sequence of Bryophytes how in a sporogonial head, as 

 thus defined, the spore-production becomes restricted in extent, and 

 relegated towards a superficial position by the formation of a central 

 sterile mass. I am ready to join Dr. Scott, in his confession of inability 

 to find anything like an intermediate form between the spore-bearing plant 

 of the Pteridophyta and the spore-bearing fruit of the Bryophyta, and to 

 agree that at the best there is nothing more than a remote parallelism not 

 suggestive of affinity ; but none the less I think we should continue to 

 search among the Bryophyta for suggestions as to the methods of advance, 

 and to have confidence in transferring these ideas across the gulf, for I 

 believe this to be both a reasonable and a promising method of study. 



( To be continued. ) 



gonial head " 



ECCREMOCARPUSES. 



Few hardy climbers are more effective when grown under suitable 

 ditions than this little family. The genus consists of two species, one «i 

 native of Chili and the other of Peru. Both are fairly hardy in most dis- 

 tricts, but quite so in the South and West of England, fine specimens being 

 frequently seen in southern gardens. I noticed recently, in the neigh- 

 bourhood of Byfleet, Surrey, a fine specimen of the best known species, 

 Eccremocarpus scaber. It was growing on a south wall, was quite 

 fifteen feet in height, and several yards wide ; its handsome flowers 

 being produced in great profusion. It was made more effective by a 

 large plant of Clematis coccinea growing near, the branches and 

 flowers of which had intermingled, and the brilliant scarlet of the clematis 

 contrasted well with the striking colour of the Eccremocarpus. The 

 flowers are produced freely from July to September, the tubular-shaped 

 blossoms being crowded on the racemes, and of a deep orange- red or 

 scarlet colour. The leaves are deep green, bi-pinnate, ovate, cordate, 

 and sometimes serrated, terminated by a conspicuous spiral tendril, and 

 altogether very ornamental. A name adopted by some authorities for 

 this plant is Calampelis scaber, and under this it was described by 

 Sweet in his "British Flower Garden," Series II., p. 3°- Eccremocarpus 

 scaber was introduced from Chili in 1824. A fine garden form is : some- 

 times met with under the name of Eccremocarpus scaber rosea, dmering 

 only in the colour of its flowers, which are salmon-rose. It is well wortn 

 cultivating, forming an agreeable contrast to the type. 



The Peruvian species, Eccremocarpus longiflorus, introduced in lo2h 



To sum up then, not only do I find that the facts in our possession, 



. *o sum up men, not only do I find that the tacts ,n our possesion, E?£S^*J^ greenish limb, whale its eaves are deep ^ree 

 including the wildest anomalies, are consistent with an antithetic theory A ^ T*> t . W ft nui te as eas y grown 



but a comparison of normal forms seems to me to support the opinion Jul ?, and . Au 8 us i> and although rare in cultivation, is quite ^easily g ^ 



that the sporophyte has appeared as the result of gradual elaboration from 

 the zygote, a fresh phase having been thus gradually intercalated in the 

 course of evolution. The idea first clearly stated by Celakovsky in 1868, 

 was developed by him in subsequent writings. I endeavoured to place it 

 on a footing of adaptation to external conditions in 1 890 ; and in 1 897 we 

 hnd btrasburger restating the position in terms almost identical with my 

 own, but upon a basis of nuclear detail which had not been dreamed of 



when the view was first propounded. Dr Scott has enthusiastically -— 



5Z^J5S»s £ »^sr# w pjafes sags msrg 



after the fruit had been set. i thoroughly syringed the trees 

 before and after, but I found that syringing with a half-pint of par attm 

 twelve gallons of water has kept them in abeyance, and the trees are in a neau j 

 state, and scarcely any of the fruit has dropped.— J. H. S. 



J — 6""M ailllUUgll I ill C 111 (.UHiiaiiu!., w n - T u|„ 



as L. scaber. The vigorous growth of the species renders them vaiuaoic 

 for covering walls, trellises, or pillars quickly. Their culture is very eas>, 

 as they thrive well in any rich sandv loam. They are readily » ncrease , u 

 by means of seeds, which, if sown in heat during February and > lar( - r f : 

 will produce plants to flower during the latter part of the same year , an 

 also by cuttings of the young growths, taken during July and Augusi, 

 provided they are kept in cold frames during the winter. 



Woking. E. SCAPLEHORN. 



strongly tmy^L^ ^ discovered appeals quite as 



such n diffi^i^fo 0f K thiS f l ^ ussi °n I have not been anxious to point out 

 such d.fticulties as beset the homologous view : all I have attempted here 



