Septembhr, 1911. 



KNOWLEDGE. 



349 



of the primary closing movement, until a sufficient number of 

 pollen-tubes have penetrated the conducting tissue and so 

 disorganised it that re-opening of the lobes is made impossible. 



The next question is, how do the entering pollen-tubes cause 

 this remarkable disorganisation of the stigmatic tissue ? The 

 entering tubes will resorb the very thin cuticle between the 

 stigmatic papillae, and they will, after some time, absorb 

 sufficient water to prevent a return to the original turgescence 

 in the stigmatic tissue, but the disorganisation of the stigma is 

 not entirely explained in this way. Evidently some chemical 

 substance or substances contained in the pollen grains and 

 tubes must diffuse into the conducting tissue, and there cause 

 disorganisation of the cells. Lutz tried the effect of watery 

 extracts of the pollen-grains. On placing a drop of this extract 

 on the stigma, the latter closed as the result of the mechanical 

 stimulus, and remained closed, also showing, after a few 

 minutes, a yellow discoloration, while the microscope showed 

 exactly the same kind of disorganisation of the conducting 

 tissue as is caused by the pollen tubes. The extract did not 

 act as a chemical closing stimulus on the stigma, however. 

 On putting a stigma into the extract it began to close gradually 

 after a few seconds, whether the stigma were irritable or not ; 

 this closing occurred in insensitive stigmas — old stigmas, and 

 stigmas made insensible by treatment with ether, for instance. 

 That is, the closing movement was due to chemical action 

 of the extract, but not to a chemical stiiinihts; the extract 

 injured the stigma cells but did not act as a stimulus to move- 

 ment. Iieyond ascertaining that the active substances 

 extracted from the pollen were not destroyed by heating to 

 100°C., Lutz made no attempt to isolate and identify them, 

 because exactly the same result was brought about by extract 

 of pollen of all kinds of other plants, and by solutions of various 

 organic and inorganic substances. 



Clearly, then, the permanent closure of the stigma lobes in 

 Miiniiliis and so on, is brought about by abundant pollination 

 with pollen of the same species, while with foreign pollen (the 

 tubes of which either do not penetrate the stigma or only do 

 so occasionally and imperfectly) the stigmas open again after 

 a time. When killed pollen-grains of the same species are 

 applied to the stigma, the latter opens again in an hour or so ; 

 the dead pollen can absorb water from the stigma tissue, and 

 of course it still contains the chemical substances which 

 injure the stigma tissue, but it cannot produce pollen-tubes 

 which are required to conduct the chemical agent to the con- 

 ducting tissue. 



Of what advantage to the plant are the movements of the 

 stigma-lobes ? t5ruk's answer was that the closing up of the 

 stigma-lobes is a protection against the germination of foreign 

 pollen. Lutz finds, however, that the pollen of some foreign 

 plants germinates almost as well on the stigmas of Miiindiis 

 and so on, as the plant's own pollen, though the foreign grains 

 do not seem able to send their tubes into the conducting tissue. 

 Hence it is not the irritability of the stigma that prevents the 

 growth of foreign pollen, but rather the specific chemical 

 characters of the pollen-grains, pollen-tubes, and conducting 

 tissue. Gartner suggested that the irritability of the stigmas 

 was necessary for the proper fertilisation of the ovules in the 

 ovary, but Lutz shows that fertilisation occurs even when the 

 stigma has become insensitive by age or rendered so artificially. 

 Of course, the germination of the pollen is favoured by the 

 closing process, which shuts the pollen grains into a moist 

 chamber, and Lutz concludes that this is the sole advantage 

 of the closing movement. When pollen is placed on the 

 stigma carefully, so that no movement results and the lobes 

 remain open, the grains germinate much more slowly than in 

 the normal case where the lobes become closed. Lutz watched 

 insects at work pollinating the flowers of Minnihts, Torcnia 

 sp., and others, and found that only very rarely did they place 

 sufficient pollen upon the stigma to make it close permanently 

 at once. 



RECENT WORK ON E X P E R I M E N 1 A L 

 MORPHOLOGY.— In 190S, Goebel gave an account of the 

 interesting results already gained by experimental researches 

 on alterations in structure induced by the action of external 



stimuli, or environment, and on the regeneration processes 

 that occur when parts are injured or removed. In this 

 work (" Einleitung in die experimentelle Morphologic der 

 Pflanzen"), Goebel emphasised the necessity of studying a 

 plant throughout its development as a condition for the under- 

 standing of its structure, pointing out examples that show 

 great diversity between the corresponding parts of the young 

 and the adult plant, and tracing these differences to causes 

 whose action could be tested by experiment. Such experi- 

 ments show that the action of the environment depends largely 

 upon the period of growth during which it has acted, and that 

 the characters of the earliest period may be retained in the 

 normally later stages, or may be reproduced in these stages by 

 the influence of an environment suitable to the earlier period. 

 For instance, the common harebell (the "bluebell" of 

 Scotland) shows rapid changes in the forms of its lea\es as the 

 result of changes in environment, these changes resulting from 

 very different causes such as diminished light, lessened supply 

 of water, increase in salts dissolved in water, and so on. 



Goebel made the generalisation that these external influences 

 act indirectly by altering the amount and kind of the food 

 formed by the plant and needed to allow of the normal course 

 of development, the external form being conditioned by the 

 vital activity of the plant and by the food supply to each part 

 of it. Goebel holds that the quality of the food supply 

 explains the nature of the parts formed. For instance, the 

 formation of stolons in the enchanter's nightshade {Circaca 

 li(tctiana) is attributable to the amount of organised food 

 relatively to the inorganic ash constituents supplied to the 

 growing-point of the stolon, being greater than that supplied 

 to the leafy shoot. Again, if potato tubers are kept at a 

 temperature not above 7°C., few roots or leafy shoots are 

 formed, but new tubers develop rapidly, though remaining 

 small; if the parent tuber is then cultivated at about 25°C., 

 leafy shoots are freely formed, the younger tubers of the new 

 growth often being changed into leafy shoots. 



Dostal [Flora, 1911) has made a large number of 

 experiments with Circaca and some other plants, and the 

 results obtained by cultivating isolated pieces of stem, each 

 piece with a single leaf and bud, are of great interest. He 

 finds that axillary buds thus treated develop into either stolons 

 (underground runners) or flowering shoots, or transitions 

 between these two kinds of shoot, according to the position of 

 the leaf and bud — whether taken from the base, apex, or 

 middle of the plant. On the other hand, if the leaf is 

 detached from the piece of stem, the bud always produces a 

 sterile leafy shoot, no matter from what region of the stem 

 it has been taken. Again, if, in experiments with a leaf and 

 bud portion, the leaf is excluded from the light, the buds 

 always grow into sterile leafy shoots. Dostal also finds that 

 the development of a sterile leafy shoot depends upon the 

 food supply ; to produce a flowering shoot the proportion of 

 organic to inorganic food must be greater than that required 

 for the formation of a stolon. On the same material (food 

 supply) conditions depend also the various characters of the 

 kinds of shoot produced — their geotropic reactions (whether 

 the shoot will grow upwards or horizontally when laid flat), 

 the lengths of the internodes ("joints") of the vegetative 

 stems and the inflorescences, the size and form and number 

 of the leaves, the number of flowers, and so on. 



Dostal's paper is of value as a careful and detailed working 

 out of the experimental morphology of a single plant, on the 

 lines indicated by Klebs and by Goebel. From the work of 

 Klebs, we know that the normal course of development in 

 any plant is only a special type of many developmental 

 possibilities ; that this normal sequence and no other is 

 usually met with must be attributed to a normal sequence in 

 the external factors. From Goebel's work, we gain further 

 generalisations, some of which have already been mentioned. 



In dealing with the production of buds in abnormal 

 situations on various plants, Goebel shows that this is closely 

 akin to regeneration, both being specially active at the 

 growing-points of axes (stem or root) and of leaves ; that 

 adult tissue in various places may, however, revert to the 



