196 . SUMMARY OF CUKEENT EESEARCHES RELATING TO 



development the embryo becomes remarkably unsymmeirical, and as the 

 cotyledons increase in size they become much curved. Shortly before 

 the seed is matured, the cambium layer of the endosperm, after it has 

 ceased cutting off starch-storing cells, divides to form a continuous 

 layer of short regular cells, forming an aleurone layer. The absence of a 

 perispermin Fagopyrum does not exclude the possibility of its occurrence 

 in other members of the Polygonaceffi, which have been several times 

 referred to in recent literature as characterized by seeds with an abuudaut 

 perisperm, and therefore (by Johnson) as somewhat closely related to 

 Piperacea3. 



Physiology, 



Nutrition and Growth. 



Parasitism.* — D. T. MacDougal has attempted an analysis of para- 

 sitism, and finds that " about half of the total number of seed-plants use 

 complex food-material derived from other organisms by mycorhizal or 

 parasitic arrangements." Dependent species undergo direct morpho- 

 logical modification and reduction, and even the fruit and seed may 

 show speciahzation. A large number of experiments have been made 

 with species capable of acting as host and parasite towards each other : 

 in particular, regenerated species were attached to the stems of desert- 

 succulents and xerophytes. Sometimes root-suckers were formed, and 

 in other cases absorption took place directly through the normal epi- 

 dermal cells. It appears that one plant cannot become parasitic upon 

 another, unless it has an osmotic pressure greater than that of its host, 

 but many causes may prevent actual parasitism, e.g. formation of wound- 

 cork, excretions, alterations in turgidity, etc. Physiological alterations 

 resulting from parasitism are of a mutative character, but there is no 

 evidence as to their fixity or transmissibility. Evolution tends towards 

 increased dependence of the parasite, and in some cases towards its 

 extinction, but evidence on this point is at present insufficient. 



In a later paper the same author f describes several cases of occasional 

 parasitism of Cactacese in the neighbourhood of the Desert Laboratory. 

 These include parasitism of species of Opuntia on the Sahuaro or great 

 tree cactus {Carnegiea gigantea) and an Acacia. Similar cases of para- 

 sitism were also induced experimentally. 



Some of the most important experimental results were obtained by 

 using regenerated cuttings of a vine, C'issus laciniata, from Mexico, as a 

 parasite. 



Is the Soil a Direct Source of Carbon ? — M. Molliard % contributes 

 a note upon some experiments dealing with the soil as a direct source of 

 carbon to green plants. Seedlings of radish were grown in soil freed 

 from gravel ; the soil was in some cases strongly heated before the 

 experiment, in other cases left in its natural state ; some tubes were 

 sealed and others left open to the atmosphere. The results were uncon- 

 vincing, and seem to show that even if carbon is obtained from the soil 

 its quantity is inappreciable. 



* Bet. Gaz., lii. (1911) pp. 249-60 (6 figs.). 



t Bull. Torrey Bet. Club, xxxviii. ((1911) pp. 473-80 (4 pis.). 



\ Comptes Rendus, civ. (1912) pp. 291-4. 



