348 



KNOWLEDGE. 



September. 1911. 



may be due to the neutralising of the acidity of the moss ; 

 that forms growing on high moors contain more acid than 

 those found on low heaths, and that the former are more 

 sensitive to lime; that on high moors there is less mineral 

 food available for the Sphagna in the substratum, and that 

 the amount of acidity and the sensitiveness to its neutralisation 

 decrease as the amount of available mineral food increases : 

 also that high moor Sphagna usually absorb more water than 

 those growing on low heaths. 



Baumann and Gully iMitt. d. K. Bayr. Moorkiiltiirsiiafalt. 

 19101 have confirmed Paul's statement that lising Spliagniiiu 

 plants give the same free acid reaction as does peat derived 

 from the dead remains o{ Spliagiiiiiii. They also show that 

 living or dried Spliagniaii plants dissolve calcium phosphate 

 in the same way as peat. From numerous experiments, they 

 conclude that the acidity of peat and of Sphagnuiii is due to 

 the presence of colloidal substances of acid character in the 

 cell-walls. 



Czapek showed, some years ago. that Spluiginiiii con- 

 tains various interesting chemical substances. ()ne of 

 these, " sphagnol." is injurious to Bacteria and thus acts as an 

 antiseptic, preserving the dead Spliaginiiii tissue from 

 destruction and helping in the formation of peat. The walls 

 of the clear water-holding cells of the leaves also yield on 

 treatment with alkah large quantities of jelly-like substance, 

 probably identical with Baumann's "Sphagnum colloids." 



SENSITIVE STIGMAS.— As is well knowu, the flowers of 

 various species of Miinuliis (monkey-flower, musk) have a 

 stigma with two flat diverging lobes, which when irritated 

 shut together so that one stigmatic (inner) surface meets and 

 presses against the other. The inner surface is the sensitive 

 part, a touch on the outside of the lobes producing no eft'ect. 

 The closing movement is instantaneous ; after from five to 

 eight minutes the lobes begin to diverge again, and in ten to 

 fifteen minutes have regained the position of rest. 



These movements, which occur in some other plants, e.g. 

 species of Torcnia and Martynia. were first described about 

 a century ago, but the only detailed account, until quite 

 recently, was that given in 1902 by Bruk, According to Bruk, 

 in Mini 111 us the lobes remain closed only it the stimulus has 

 been caused by placing on them specific pollen {i.e.. pollen 

 from the same flower or from a flower of the same species), 

 and soon open and remain open if foreign pollen (i.e. pollen 

 from a different species of plant) is used. He also stated that 

 in Torcnia fournicri the lobes will open again and remain 

 open if pollen from the two shorter stamens of the same 

 flower has been placed on them ; this point is referred to 

 below. 



Oliver, in 1887, showed that the stimulus is transmitted 

 from one lobe to the other ; if one lobe is prevented from 

 moving leg. by cementing it to the corolla), a touch on its 

 inner face still provokes movement in the other lobe. He 

 also showed that the transmission takes place through the 

 ground tissue of the lobes, probably by means of intercellular 

 protoplasmic connexions, since transmission occurs when the 

 vascular bundle is cut through. 



Lutz has made a thorough investigation of irritable stigmas 

 iZeitschrift fiir Botanik, 1911). The ground tissue of each 

 stigma lobe is sharply divided into two zones, the inner zone 

 foruiing the conducting tissue (down which the pollen tubes 

 pass) and the outer consisting of compact parenchyma; the 

 epidermis of the outer side is strongly cutinised and has no 

 papillae, that of the inner side has very thin cuticle and is 

 produced into long hairs which receive the stimulus of contact. 

 The movement of the lobes is brought about by a sudden 

 lowering of the osmotic pressure, and diminution in volume of 

 the whole ground tissue. The diminution in volume is not 

 uniform, but very different on opposite faces of the stigm.a 

 lobes, being in fact about twice as great on the inner face as 

 on the outer face. By artificially withdrawing water by 

 osmosis there can be produced a closing together of the lobes 

 similar to that resulting from stimulation. 



Lutz carefully investigated the results of pollinating the 

 stigma of Mimuhis cardinalis. and so on, with pollen of the 

 same species and also with foreign pollen. He tried the effect 

 of placing pollen in different quantities on the inner surface 

 of the stigma lobes — on one small area, on about half of the 

 inner surface, on the entire inner surface. If little pollen is 

 used, whether of the same species or not, the lobes at once close, 

 then open again in about ten minutes. With more pollen, 

 enough to cover about half the inner face of the stigma lobe, 

 the lobes open again after about ten minutes ; but if the pollen 

 used is of the same species, a second closing movement begins 

 after from two to three hours and lasts for about twenty 

 minutes. If, however, the pollen is carefully placed on the 

 stigma to avoid mechanical shock, the second closing movement 

 occurs as usual after between two and three hours, although 

 the first (contact irritability) movement is not made. When a 

 large amount of pollen is used, the whole inner face of the 

 stigma being covered, the first instantaneous movement occurs 

 as usual, owing to the mechanical shock stimulus in applying 

 the pollen, but the lobes remain closed, never diverging again. 

 If the stigma is loaded with foreign pollen, it closes at once 

 as usual, and remains closed for two or three hours, but then 

 opens again. 



In these interesting experiments with small, medium, and 

 large quantities of pollen, Lutz worked chiefly with Miinulus 

 cardinalis. and used in each case (1) pollen from long 

 stamens of same species, (2) pollen from short stamens of 

 same species, and (3) pollen from various foreign species — 

 Snapdragon. Plantain, and others. His results show that the 

 permanent closure or the reopening of the stigma lobes is 

 determined in the first instance by the quantity of the pollen 

 applied — a large quantity causes permanent closure, a smaller 

 quantity does not : and. secondly, by the origin of the pollen 

 used — a full load of pollen of the same species results in 

 prolonged closure, foreign pollen in closure for a few hours at 

 most. In opposition to Bruk's results, Lutz finds that in 

 Torcnia the anthers of the short stamens dehisce later than 

 those of the long stamens ; that pollen from undehisced 

 anthers is not capable of germinating ; and that when such 

 pollen is placed on the stigma, the latter opens in ten to fifteen 

 minutes after the usual closure. 



Lutz proceeds to analyse the two closing movements which 

 result on the placing of suitable pollen on the stigma. The 

 instantaneous first movement is purely the result of the 

 mechanical stimulus. Bruk explained the permanent closing 

 of a pollinated stigma as being due in part to the pollen grains 

 abstracting water from the stigma tissue, and thus preventing 

 this tissue from returning to its original state of turgescence. 

 This explanation is supported by the observation of Lutz that 

 the stigma only remains closed when a large amount of pollen 

 has been placed on it. Lutz also noted that sometimes, even 

 with a full load of pollen, the stigma lobes after from one to 

 three hours opened again, doubtless recovering turgescence by 

 being supplied with water from the ovary and style — but this 

 re-opening would not. of course, interfere with fertilisation, 

 since the pollen had had ample time to germinate and send the 

 pollen-tubes into the conducting tissue. 



Lutz found that not only specific and foreign pollen, but 

 also such substances as dry sand and powdered starch could 

 cause prolonged closure of the stigma lobes. On the other 

 hand, wet pollen caused only temporary closure (for ten to 

 fifteen minutest ; only dry pollen could produce prolonged 

 closure. The permanent closure, resulting simply from the 

 prevention of the automatic re-opening movement, is due to 

 ( 1 ) the absorption of water by the pollen grains and their 

 germinating tubes, and (2) the chemical effects of the growing 

 pollen-tubes on the conducting tissue of the stigma lobes. If 

 either of these conditions be absent, the stigma lobes open 

 again after a shorter or longer time. Both conditions are 

 fulfilled by the germination on the stigma of the pollen of the 

 same species. This portion of Lutz's results may be 

 summarised by stating that the stigma remains closed only if 

 the pollen grains and the germinating pollen-tubes can, by 

 abstraction of water, prevent the return of the original osmotic 

 pressure in the stigmatic tissue, and the consequent reversal 



