EVOLUTIONARY TENDENCIES AMONG GYMNOSPERMS 4^7 



as we know them, on the other hand; a gap which there seems to be 

 small probability of filling up with intermediate forms. In this pres- 

 entation the only thing possible is to take the ovules of the paleozoic 

 forms as representing the oldest known ovules, and to note the general 

 changes that have occurred since. 



To select the most primitive type of ovule from among the paleozoic 

 forms that have been investigated is impossible, unless it is assumed 

 that those ovules which are most unlike the modern ones represent 

 the most primitive type. This may or may not be true, but it is the 

 only available criterion; and by using it, we obtain the following result. 

 The oldest ovule had a single integument entirely free from the nucel- 

 lus; in testa-formation this integument differentiated into three layers, 

 the outer fleshy, the stony, and the inner fleshy; the ovule was sup- 

 plied with two sets of vascular strands, the outer set traversing the 

 outer fleshy layer, and the inner set traversing the peripheral region 

 of the nucellus; and the beaked tip of the nucellus broke down more 

 or less completely within the firm and resistant epidermis to form a 

 pollen chamber. If these are really the features of the most primitive 

 known ovules, the changes become very apparent, and they represent 

 general tendencies, for they appear in every phylum. 



In the first place, the integument and nucellus, instead of remain- 

 ing separate, develop separately only in the region of the nucellar 

 beak. So early was this change that it probably represents the con- 

 dition of the majority of paleozoic ovules, a condition which has per- 

 sisted ever since. The method of development is very evident, the 

 integument appearing first as a distinct annular growth about the 

 base of the young nucellus, but later its basal meristematic zone 

 becoming indistinguishable from that of the nucellus. In aU proba- 

 bility the change was brought about by the earlier appearance of the 

 integument, and the result has been more or less variability in the 

 amount of freedom from the nucellus. 



The three-layered testa persists remarkably throughout gymno- 

 sperms, varying chiefly in the amount of development of the outer 

 fleshy layer. The stony layer is always strongly developed, and at 

 the maturity of the seed the inner fleshy layer always forms for it a 

 papery lining. A strong development of the outer fleshy layer, result- 

 ing in fleshy seeds, continues throughout the cycadophyte phylum 



