TRANSACTIONS OF SECTION K. 835 



2. Foliolar Theori/. 



On this theory the ovule is homologous with a leaflet of a carpel, viz., the 

 integuments with the terminal and two lateral segments of the leaflet, and the 

 nucellus with an emergence borne on the upper or ventral surface of the former ; 

 the nucellus is thus directly homologous with an eusporangium, such as that of 

 Angiopteris. Primitive position of nucellus or sporangium is terminal to leaflet, 

 as is case in normal ovule, where the homologue of leaflet of carpel takes the 

 form of one or two urceolate envelopes. Case of Ferns where sporangium is 

 usually borne on lower sui-face of leaflet is an instance of progressive metamor- 

 phosis from the primitive condition ; here the green leaflet, or its receptacular 

 representative, is the homologue of the outer integument of the ovule, and the hi- 

 dusium of the inner integument of the latter. The leptosporangium of most 

 ferns is the result merely of the ultimate subdivision of the eusporangium, and is 

 homologous with a trichome. The abnormalities resulting from the metamorpho- 

 genesis of the parts of the ovule are the decisive and only reliable sources for 

 determining the true morphology of the ovule. Chief propounders of this theory 

 are Bronnniart (the founder, 1834) ; Cramer (18(34), Celakovslcy (1874-1900), 

 Eichler (1875), Warming (1878). Of these, Celakovsky is responsible for the 

 formulation of the theory as above summarised. 



3. Sui generis Theory. 



This theory holds that the sporangium, with its homologue the nucellus, is an 

 organ sui generis, and cannot be included under any of the morphological categories 

 of stem, root, leaf, or trichome. The integuments are new structures arising on the 

 sporangium or nucellus. The abnormalities are of no permanent value for deter- 

 mining the morphological relationships of the parts concerned. The chief up- 

 holders of this view are Sachs (1874), Goebel (1887), Schmitz (1872), Strasburger 

 (1879), Bower (1894). 



9. The Histology of the Sieve Tubes q/'Pinus. By A. W. Hill, 



The sieve tubes of Gyninosperms have been previously investigated by De 

 Eary, Janczewski, Russow, Keinitz-Gerlofl", and Strasburger especially with refer- 

 ence to the structure of the sieve plate and the mode of communication between 

 adjoining sieve tubes. 



The present researches have proved that the results obtained by Russow are, in 

 the main, correct ; for it has been found, as he describes, that the mature sieve 

 plate is traversed by groups of caUus rods, which are interrupted at the middle 

 lamella by median nodules, and that each callus rod contains from three to seven 

 striae — or spots if examined in surface view — which are strings of slime. 



With questions of development Russow was not very successful, and it is with 

 them that the chief interest of the research lies. 



The youngest sieve plates or pit-closing membranes, which could be examined, 

 showed ' connecting threads ' like those in ordinary tissue ; but in the so-called 

 ' boundary cells' — i.e., the youngest thick-walled sieve tubes — a change takes place, 

 namely, the appearance of the callus. Callus first appears on one surface of the 

 sieve plate, at the places where the groups of ' connecting threads ' occur, and it 

 gradually spreads as a rod along a group of the threads to the middle lamella; a 

 similar change then takes place on the otJier side of the lamella. The lamella 

 itself, however, is not converted into callus, but a refractive median nodule appears 

 separating the two portions of the callus rod. 



Accompanying this change the protoplasmic threads become converted into 

 slime strings. A similar state of things obtains in part with the sieves between 

 the sieve tubes and the albuminous cells. 



The changes just described ai-e without doubt due to the action of fer- 

 ments, which travelling along the threads convert them into slime strings and 

 at the same time alter the cellulose portion of the pit-closing membrane in their 



