May 5, 1893.J 



SCIENCE. 



241 



The latter sort of bleeding is necessarily delayed until growth is 

 about to begin, and is checked as soon as the foliage is sufficiently 

 expanded to begin evaporation. 



A bleeding similar to the last takes place at the hood-like tips 

 of grass leaves, where the skin is nearly always ruptured. The 

 little drops of water which accumulate here are commonly mis- 

 taken for dew, but are merely droplets exuded from the interior 

 of the leaf, because the falling temperature of the air toward 

 evening has diminished the evaporation from the leaves, while 

 the roots in the warm soil are still absorbing water, and conse- 

 quently producing an internal pressure. The movement of water 

 in these cases of bleeding, it will be seen, is necessarily toward 

 the point of exit, which may be above or below the point at which 

 the pressure arises. 



Secretion of Nectar. 



A third sort of movement of water is that which takes place in 

 the nectaries of flowers and leaves. The flowers of our common 

 linden, forexample, secrete a considerable quantity of sweet fluid, 

 which is sometimes miscalled "honey," but is properly known 

 as nectar. Honey, by the way, is nectar after it has been digested 

 by the bees. At certain points in the flower there are groups of 

 cells whose special business it is to withdraw water from the parts 

 below, and filter it through their outer walls, after having added 

 to it the materials which make it sweet. The movement of water 

 in this case is extremely limited. 



The Transfer of Food. 



The last movement of water of which I shall speak is of those 

 solutions which contain the food of the plant. These materials 

 are not those absorbed from the soil, or gathered directly from 

 the air, but they are the substances which have been manufactured 

 by the leaves out of the materials obtained from the soil and 

 from the air. Since these foods are put together in the leaves, 

 necessarily the movement of water containing them in solution 

 must be in a difi'erent direction from that which supplies the 

 evaporation. The materials thus manufactured in the leaves 

 must he carried either to those parts which are growing or to 

 those places in which they are to be stored for future use. It is 

 manifest at the first glance, therefore, that the direction of the 

 movement must be in general inwards from the leaves, and, since 

 the roots require for their nutrition a considerable amount of 

 these substances, there must be a very decided downward move- 

 ment to supply them. 



Now it is plain that these solutions of food must keep out of 

 the way of those portions of the water which are chiefly to supply 

 the evaporation from the leaves. We have seen that the latter 

 travel in the sap wood. The food currents, however, travel almost 

 exclusively in the inner parts of the bark. You will therefore 

 understand why stripping off the bark, or even cutting it, ensures 

 the death of the tree eventually, even though the leaves remain 

 long unwithered, since the roots depend upon the food formed 

 by the leaves, they perish when severed from their base of sup- 

 plies. 



The movement of the evaporation stream is relatively rapid. 

 The movement of this food current is relatively slow. We do 

 know something of the mode of movement of these food currents. 

 They are apparently brought about through the process known as 

 dififusion, or osmosis, and are therefore necessarily slow. The 

 cause of the movement is practically the same as that for the 

 movement of oil in the lamp-wick, although it is by no means 

 by the same method. The oil in the lamp-wick travels upward 

 because at the top it is being destroyed as oil by reason of the heat 

 of the flame. So the direction and existence of the current of 

 water carrying food is because the various substances dissolved in 

 the water are being altered at the place of growth or storage into 

 nesv materials. The commonest of these food substances is sugar, 

 and at the growing point of the stem, for example, the sugar is 

 being constantly destroyed as sugar and is being converted into 

 cellulose or protoplasm or some other material. So long as that 

 alteration is going on, just so long will the sugar particles move 

 toward that point. 



But 1 must not impose further upon your patience. I have 



tried to sketch very briefly, and only in outline, the different 

 movements which the water in the plant is undergoing. I have 

 said nothing of the extreme variety of materials which may be 

 found in this water in different plants, or even the variety found 

 in the same plant at different times, but have endeavored merely 

 to show you that there is going on constantly in the living tree a 

 series of molecular and mass movements, of which too few people 

 have any conception. To our imperfect knowledge let me hope 

 that some of you may contribute facts which shall enable us some 

 day to explain the many things which are now obscure. 



NOTE ON THE SEA.-URCHIN SKELETONS. 



BT HENRT LESLIE OSBORN, PH.D., ST. PAUL, MINN. 



In looking through the drawings of students in the freshman 

 class of the aboral ring in Strongylocentrotus droehachiensis- 

 from Portland, Maine, I came upon one which I at first supposed 

 to be erroneous, but which, on comparison with the specimen, 

 proved the specimen to be exceptional. The usual arrangement 

 of the genital and ocular plates in this species is shown in Fig. 7 

 of our cut. The five genital plates and two of the oculars border 

 upon the anal ring, the three remaining oculars being shut out by 



IDoTOc\3.at"»s 



the contiguity of the enlarged genitals. The arrangement found 

 by exception is figured at 8. It consists in the exclusion of four 

 oculars from the aboral ring, so that only one gets a share in 

 forming the border. I seized the occasion to look into the cases 

 of about fifty specimens which happened to be on hand in the 

 laboratory, and found that the case of Fig. 8 occurred twice in 

 that series and that all others were like Fig. 7, which is normal, 

 and which I have observed in many more than fifty different 

 specimens at difi'erent times. It is interesting to note that in 

 Fig. 132 of Agassiz's "Seaside Studies," page 103, a drawing of a 

 specimen of this species is given, in which three ocular plates 

 border the aboral ring, and in which the plates are thus quite 

 symmetrical. This must be of very exceptional occurrence, for 

 I have never met it in the many specimens I have seen. I should 

 be very glad to know if it has been at all generally observed. 



In connection with the case of Strongylocentrotus, it is inter- 

 esting to examine the aboral ring of other regular echinoids. In 

 Diadema (Fig. 1) the ring is perfectly regular, with five genitals 

 and five oculars of equal size; in Arbacia (Fig. 6) it is equally 

 regular, but with five large genitals, which form a ring about 

 the aboral area, and exclude from it wholly the five small geni- 

 tals. In Dorooidaris (Fig. 3) the ring is nearly regular, four 

 oculars barely reaching the ring, the fifth being shut out. In 

 Hipponoe (Fig. 4) the case is nearly as in Diadema, one ocular, 

 however, not reaching the ring. In Echinometra (Fig. 9), a very 

 elongate urchin, but not elongate to the plane of the madreporic 

 plate, the five oculars do not any of them rr ach the ring and the 



