158 



HORTICULTURk. 



Augrust 18, 1906 



Do Orchid Roots Gather Vapor of Water from the Air? 



In the issue of Horticulture for July 7, my friend 

 Mr. James Hutchinson, in an article on Oncidium 

 sphacelatum Lindl, makes mention of the supposed ac- 

 tivity of the roots in absorbing vapor of water directly 

 from the atmosphere. Thus is raised again a question 

 upon which I should like to say a further word. 



The impression that orchids possess a special power 

 of absorbing water vapor— that is, the uncondensed or 

 gaseous form of water— from the air, arises from the 

 observation that orchids are '"air plants," growing 

 perched on limbs and trunks of trees or on rocks where 

 their roots cannot roach any constant water supply in 

 the liquid form, and from the further observation that 

 these plants apparently continue to thrive- they cer- 

 tainly grow for a time— in the greenhouse when they 

 are not watered even for some months. The appear- 

 ance of new shoots on these plants seems to be evidence 

 of access of water in some form, and if this is not 

 available in liquid form then the source— it is reasoned— 

 must be the gaseous water always present in the atmos- 

 phere. The suspicion that the roots are active con- 

 densers and absorbers seems to be verified by the struc- 

 ture of orchid roots, which possess several to many lay- 

 ers of empty cells in the spongy velamen, or covering, 

 whose walls would seem to be well suited for the work. 

 Furthermore, there is high authority in the older scien- 

 tific literature for the theory of direct vapor absorption 

 by these aerial roots. Kerner, in his great Natural His- 

 tory of Plants, for example, dwells at length upon the 

 fine adaptation of cell-structure to this work of absorp- 

 tion, which absorption he considers to be proved by the 

 experiments of Sachs and others. An extract may be 

 given. "But these roots also possess the power ot con- 

 densing the aqueous vapor contained in the air. They 

 act upon the moist air in which they are immersed in ex- 

 actly the same way as spongy platinum or any other 

 porous body. If the aerial roots of Oncidium sphace- 

 latum are transferred from a chamber full of dry air to 

 one full of moist air, they take up in twenty-four hours 

 somewhat more than 8 per cent, of their weight of 

 water; those of Epidendrum elongatum absorb 1] per 

 cent., while in the case of many other tropical orchids 

 the amount thus imbibed is doubtless much more con- 

 siderable still." Nevertheless, I believe that the roots 

 and other parts of the orchid plant are quite devoid of 

 any special powers of gathering vapor — i. e., gaseous and 

 uncondensed water — from the air. The defect in all the 

 older experiments was the failure to guard against dew- 

 formation in the moist chambers used for exposing the 

 roots. The older botanists were as a rule poor phyiicists. 

 Everyone familiar with physical experimentation knows 

 that a closed chamber of air safurated with water vapor 

 at a given temperature will experience precipitation of 

 dew from the contained air, if the temperature lowers 

 even a fraction of a degree. Now dew is liquid water, 

 and if this is the source of the roots' gain in weight. 



the matter is cause for no special wonder, since roots 

 generally absorb liquid water. 



At the Ames Botanical Laboratory extensive experi- 

 ments have been made, published in full some time 

 since, in which the source of error of the older experi- 

 ments was avoided. The chamber used was never quite 

 saturated by moisture, the humidity ranging from 90 to 

 95 per cent., as recorded by the hj-grometer. No dew 

 could be formed under the conditions maintained. And 

 in such a high-humidity chamber orchid roots from the 

 genera Angraecum, Brassavola, Brassia, Burlingtonia, 

 Cattleya, Cypripedium, DendroViium, Epidendrum, Lae- 

 lia, Masdevallia, Maxillaria, Odontoglossum, Oncidium, 

 Peristeria, Scuticaria, Selenipedium, and Vanda, were 

 shown by most careful weighings on delicate scales to 

 lose weight steadily until dessication resulted in death 

 after some days or weeks. These experiments were done 

 with cut roots, the cut ends sealed with wax. If our re- 

 sults are not conclusive on account of the roots being 

 cut off from the plants, neither are the old experiments 

 of Sach, Duchartre and others, which were likewise 

 done with cut roots. But to me, after considering the 

 vital relations attentively, the experiments are entirely 

 conclusive, to the effect that the spongy sheath of orchid 

 roots has no special powers of condensing water-vapor, 

 and passing this water along to the plant for use Not 

 to let the question rest in dispute, however, I made ex- 

 periments with whole plants, the roots of which were 

 suspended in the moist chamber, while the stem and 

 shoot were enclosed in a separate receptacle The 

 plants, with the receptacle for the top, were we-ghed 

 from time to time. It was proved that thus conditioned 

 the roots furnished the plant with no water, although 

 the plants in the course- of more than two months made 

 some new growth. This must have been at the expense 

 of water already within the plant body at the beginning 

 of the experiment. 



About the time our experiments were going on, 

 Nabokih, in Germany, tested eleven species from nine 

 genera in saturated air kept at a constant temperature 

 and found no absorption of vapor of water, exct'pt a 

 minute quantity in one root. His conclusions and mine 

 agree. 



After all, there is no reason why orchids in their na- 

 tive localities should seek water vapor ; because the rains 

 and dews in the tropical mountain forests are heavy for 

 long periods together. In the Andes, for example, even 

 in the dry season there are very abundant dews at night, 

 so that the orchid roots become saturated with water 

 and the plant gets all it needs for daily use. This is 

 the characteristic condition for the great orchid re- 

 gions. Speaking from experience in the Orient, Goebel 

 says: "One needs to have made but a single morning 

 excursion in the mountain region of a tropical forest in 

 order to understand how wet the woods are even after a 

 rainless night, and to see that the root-sheath cf the 

 orchids is capable of taking up a great quantity of 

 water from the dew as well as from the rain." Goebel, 

 one of the keenest observers, does not agree with the 

 view that vapor-absorption is an important function of 

 these roots. 



'^i^^L^^t, 



Ames Botanical Laboratory, N. Boston, Mass. 



