K,— BOTANY. 191 



a regular layer, serve as the basal cells, or oogonia, of the secidium. The 

 binucleate condition now supervenes ; in some species a vegetative 

 nucleus migrates into each basal cell, in others the basal cells unite in 

 pairs, and jointly cut ofE a binucleate structure which will form 

 secidiospores. 



The secidium may be regarded as a sorus, or group, of spore-producing 

 cells, comparable to the sorus which gives rise to the uredo- or teleutospores ; 

 it is, however, the product of the gametophyte and the scene of transition 

 from the haplo- to the diplophase. 



On the same mycelium of xminucleate cells which bears the young 

 secidia appears a fourth type of sorus, the spermogonium or pycnidium, 

 consisting of a layer of narrow filaments from the tip of each of which a 

 series of small oval cells is budded off. These cells, the spermatia or 

 pycnospores, each possess a large, dense nucleus, scanty cytoplasm and 

 apparently no reserve material ; they have never been seen to form a 

 mycelium, though they can be induced to undergo a form of yeast-like 

 budding in solution of honey or sugar, and Professor Robinson informs me 

 that he has observed the same thing under natural conditions. As long 

 ago as 1882 Rathay^^ called attention to the attractive characters of the 

 spermogonia, their scent in many cases, their sugary secretion, and the 

 bright colours imparted to the neighbouring host tissue ; he suggested 

 that insects were responsible for the distribution of the spermatia. The 

 function of the spermatia, however, has long been a puzzle ; as conidia 

 they were oddly constructed, as antheridia there seemed little opportumty 

 for them to reach the basal cells of the secidium ; in either case they 

 appeared to be vestigial. 



A new aspect has recently been given to this problem by two letters 

 to ' Nature,' -^^ describing the experiments of J. H. Craigie on Puccinia 

 Helianthi and Pwxinia Graminis. In the former species he found that, 

 when basidiospores are shed on the leaf of the sunflower, spermogonia 

 appear in about eight days. Ten or eleven days after sowing, when 

 mycelia from different infections overlap, aecidia are found in fifty per 

 cent, of the cases. The remaining infections, whether simple or compound, 

 do not produce aecidia for three weeks ; later nearly half of them do so. 

 This seems a straightforward case of heterothallism ; the production of 

 spore fruits is induced or stimulated by the association of two mycelia 

 of presumably different strain, but, as in the Hymenomycetes, fructifica- 

 tions may more slowly develop without such encouragement. In his 

 second letter Craigie adds a most interesting point ; observing the visits 

 of flies to his spermogonia, he was reminded of the old suggestion of their 

 function as distributors, and was induced to mix the spermatia from 

 several spermogonia and apply the material to his infections. In nearly 

 every case aecidia were the result. The inference is drawn that the foreign 

 spermatia served as a stimulus to development. Craigie regards the two 

 heterothallic strains as of different sex, and the spermatia as conidia. 

 It is possible that they play the same part as the oidia of Coprinusjimetanus, 

 but unfortunately microscopic details are not available. It will be most 

 interesting to know how the spermatium, after landing on the epidermis 

 of the leaf, penetrates to the endophytic mycelium of the rust. Such 

 knowledge should decide its antheri(fial or conidial function. 



