324 



NA TURE 



[August 3, 1905 



the leaf-stalk. Four leaves were immersed, two (D) 

 having their leaf-stalks darkened with tin-foil, while 

 the stalks of the other two (l) were exposed to oblique 

 light. After three or four days the D leaves showed 

 no signs of taking up the light-position, while the 

 two L leaves showed well marked curvature towards 

 that position. The experiment is of importance, since 

 it shows that immersion in water does not prevent 

 heliotropic curvature by interfering with respiration 

 or by depressing the energy of the plant in anv other 

 way. The only explanation seems to be that of the 

 author, viz. that in the leaves (d) with darkened 

 stalks the lens-like epidermic cells of the leaf-blade 

 are the only organs of light-perception, and thev 

 being thrown out of action by the presence of water, 

 perception (and therefore curvature) is absent. 



Experiments of the same type were made with a 

 like result on Ostrya vulgaris and Begonia discolor. 

 It is to be regretted that the light-perceiving organs 

 of such leaf stalks as were sensitive to light under 

 water were not investigated. 



A striking result was obtained with Trop^olum 

 (p. 92). The leaves of this plant are unwettable, and 

 when immersed remain coated with a silverv mantle 

 of air. The waxy layer, which gives this quality, mav 

 be removed by painting the surface with ' dilute 

 alcohol without injury to the leaves. The result of 

 immersion is that the normal leaves protected bv a 

 layer of air react normally to oblique illumination, 

 whereas the wettable leaves have lost the power of 

 so reacting. This interesting result suggests to the 

 author a new function for the waxy " bloom " of 

 leaves, i.e. that it saves them from being blinded by 

 a shower of rain. This theory he extends to velvetv 

 leaves, the strongly papillated epidermic cells of which 

 stand up like islands when the surface of the leaf is 

 wetted (p. 65). This is a striking fact in relation 

 to the distribution of velvet-leaved plants, which 

 are especially common in damp tropical regions. 



Another section of Haberlandt's evidence depends 

 on the existence of highly specialised lenses. One of 

 the most curious is that of Filtniiia Vcrschaffeltii 

 (AcanthacccE), shown in Fig. i. Here we have a 



-^1^^^^.^^''-^^ 



Fig. I.— Ocellus of Fittonia Verschaffiltii. 



dwarfed, two-celled trichome, of which the apical cell 

 has the form of a biconvex lens. In this case there 

 is a division of labour, the light focused bv the lens- 

 cell being perceived bv the large basal cell. Direct 

 experiment shows that, as might be expected, paint- 

 ing the leaf with water in no way interferes with the 

 effect, since the lens is raised above the laver of wet. 

 Similar ocelli occur in Jmpatiens mariaunae, and 

 here, as in Fittonia, it is interesting -to note that the 

 ordinary epidermic cells, among which the ocelli 

 occur, are markedly bad lenses. 



Quite a different type of lens occurs in Campanula 



NO 1866, VOL. 72] 



persicifolia ; here (Fig. 2) the formation of a spot of 

 light does not depend on the form of the epidermic 

 cell as a whole, but on the existence of a lens-shaped 

 silicified region in the outer wall of the cell. These 

 structures only occur in perfection in a shade-loving 

 form of the species, where they were noted by 

 Heinricher, who was unable to suggest a function 

 for them. Direct observ-ation proves that they are 

 highly effective lenses. Similar organs are found in 

 Petraea volubilis. We must pass over a number of 

 other interesting specialised organs, but it is of 

 importance to note that whenever ocelli occur they 



Cellus of Caiiipanitla persici/otia. 



are to be found on the upper, and not on the lower, 

 surfaces of leaves. It is also particularly interesting 

 to find that ocelli tend to occur especially near the 

 edges of leaves, i.e. just in those regions where the 

 amount of movement, corresponding to curvature 

 through a given angle, is greatest. 



The author has once more earned the gratitude of 

 his fellows by his suggestive discoveries and specu- 

 lations He must be allowed to have made out a 

 strong case for his theory, but he would be among 

 the first to grant that more work is needed before it 

 can be considered as completelv established. 



F. D. 



RECENT PUBLIC.iTIOX.'i /.Y .AGRICULTURAL 

 SCIENCE. 



EVERY civilised State has recognised a special 

 duty towards its farmers in the way of 

 endeavouring to secure them against the purchase of 

 adulterated manures, fr.iudulent feeding stuffs, and 

 dead or impure seed, but different countries have 

 taken very various means towards securing the de- 

 sired end. The United Kingdom, probably because 

 its representative farmers are men of substance, rather 

 holds by the old caveat emptor maxim, and is content 

 with providing the farmer with a machinery for 

 getting an analysis below cost price, but a machinery 

 sufiiciently cumbrous to ensure that no one sets it 

 in motion. Other nations, less intent, perhaps, upon 

 a plausible case in Parliament, and more concerned 

 in getting the thing itself done, have devised various 

 systems of controlling the trade in such materials, so 

 as to ensure that the smallest farmers shall be sup- 

 plied with seed or manures reaching a certain 

 standard of purity. The laws and methods adopted 

 for securing such a control in the various States Prof. 

 Giglioli passes in review,' giving an account of the 

 testing stations, the regulations, the fees, and even 

 notes on the working details employed in the labor- 



1 "Ci-ncim", Mangimi, Sementi, &c., Commercie, frodi, e repressione 

 delle fiodi, Spccialmenle in Italia." By Italo Giglioli Pp. xvi + 759. 

 (Rome : Annali d'.Agricoltuia, 1Q05.) 



