Apeil 22, 1910] 



SCIENCE 



633 



the amount of tomentum present on the podetia. 

 This, being a variable character, has led to much 

 confusion. It is suggested that the types of 

 cephalodia, being mutually exclusive, may furnish 

 a basis for distinguishing the species. Stereo- 

 caulon paschale has conspicuous, gray cephalodia 

 containing Stigonema; while in S. tome^itosum 

 they are minute, deep green and with an alga of 

 the Nostoe type. In only one case, out of 103 

 specimens studied, were both types found on the 

 same plant. A statistical study based on the 

 development of tomentum, the position of the 

 apothecia and the type of cephalodia shows that 

 cephalodia are present in about 90 per cent, of the 

 specimens, being more constant in their occurrence 

 than apothecia, and that they are correlated with 

 the development of tomentum to a sufficient degree 

 to warrant their use as a criterion for separating 

 the two species of Stereocaulon named. It may be 

 added that S. alpinum Laur. has the same type 

 of cephalodia as S. tomentosum; while S. coral- 

 loides Ft. and S. denudaium Flke. have the pas- 

 chale type. 

 Cell and Nuclear Division in Closterium: B. F. 



LUTMAiSr. 



Closterium, like many of the algs, is only 

 found in division at night from 10 P.M. to 4 A.M. 



The iirst external sign of division is a pinching 

 in of the chromatophore about two thirds of the 

 way from either tip. No external change is visible 

 in the nucleus at this time. Later, the nucleus 

 apparently disappears and across the middle of 

 the Closterium, in the place formerly occupied by 

 it, appears a broad granular band. The cell wall 

 now begins to grow in across the center of this 

 band separating the two halves. In stained whole 

 mounts the two nuclei resulting from division can 

 now be seen making their way back immediately 

 under the plasma membrane to the point where 

 the chromatophore is being pinched in two, in 

 either half. The two halves now break apart, and 

 the new ends are rounded out at first, but rapidly 

 grow and become pointed, making the two halves 

 of the new Closterium symmetrical. The entire 

 process takes about four to sis hours, as few 

 asymmetrical ones are found at 8 A.M. 



The chromosomes are formed from a spireme 

 whose origin is in the fine reticulum around the 

 compound nucleole. There are about thirty to 

 forty of them, slender rods. They arrange them- 

 selves on the equatorial plate of a spindle with 

 broad poles, similar to that described for 8piro- 

 gyra. In the reconstruction stages they seem to 

 unite end to end to form a dispireme. They spin 



out and become fainter and the compound nucleole 

 reappears. There is no evidence that the chromo- 

 somes have their origin from the nucleole. The 

 two nuclei move away from each other around the 

 chromatophore, between its ridges, to take their 

 place at the middle of the new halves. 



Cell division is by the growth inward of kino- 

 plasmic material which lays down the new wall. 

 The wall cuts across the cell at right angles to the 

 side walls. The central spindle fibers disappear 

 and have nothing to do with its construction. 



Corallorhima and Mycosymhiosis : Ben J. C. Gbuen- 



BEBO. 



Several species of Corallorliiza store starch in 

 the rhizome; this is secondary starch, that is, it 

 is derived from organic materials in the soil or 

 humus and is not the direct result of photosyn- 

 thesis on the part of the plant. Stomata are 

 present in all parts of the epidermis; these are 

 probably active and must be concerned with gas 

 exchanges involved in respiration. The trichomes 

 and epidermis of the rhizome serve for absorption 

 of materials from the soil. It is not necessary to 

 assume that any species of Corallorhiza is de- 

 pendent upon its fungal symbiont for its nutrition. 

 The symbiosis is indeed a constant character of 

 the species examined, but it probably results from 

 the habit of the fungus; it is at any rate not 

 necessary to assume that it is obligatory for the 

 maintenance of the orchid. The fungus may be 

 of assistance to the orchid by furnishing condi- 

 tions favorable to the germination of the latter's 

 seeds; but it is not improbable that other condi- 

 tions may also stimulate the seeds to germinate. 

 It is not certain that the fungus is indispensable 

 to the orchid in this connection. The permanent 

 association does not seem necessary for the ger- 

 mination since there are no hyphal connections 

 between the rhizome and the inflorescence. The 

 infection of the rhizome takes place about the 

 time of germination. Hyphae traversing the 

 trichomes are on their way out; these connections 

 may serve the fungus as means of propagation, 

 but need not be assumed to be of use to the orchid 

 in nutrition. The " digestion " of hyphal masses 

 within the cortical cells may be considered as a 

 means for preventing the spread of the fungus to 

 the point of injuring the orchid; it need not be 

 assumed to be necessary for the nutrition of the 

 orchid. The fungus is probably an internal sapro- 

 phyte. The myoosymbiosis may have different 

 significance in other families of plants. There is 

 in preparation a " host " index and bibliography 

 of all mycorhizas that have been described. 



