210 SUMMARY OF CURRENT RESEARCHES RELATING TO 



fern stem divides, the resulting growing points may continue to grow 

 simultaneously (dichotomy), or one of them may at once become dormant, 

 while the other continues its growth in the line of the original axis 

 (monopodial axis) ; or, thirdly, the dormancy of one of the growing 

 points may be delayed for a variable period. The second case is derived 

 from the first and involves the sacrifice of one branch for the good of 

 the other ; the third case is intermediate. But there is no parallelism 

 between the evolution of the modes of branching on the one hand and 

 the evolution of the plants themselves on the other ; the two processes 

 have been independent of each other. Dichotomy still persists among 

 the higher ferns, while some of the most primitive ferns (Ophioglossacese) 

 show an advanced monopodial type of branching. The monopodial 

 type of branching has been derived from the dichotomous by a process 

 of retrogressive evolution in the basipetal direction, involving the succes- 

 sive intercalation, at the base of the branch, of a series of stages, each 

 morphologically less complex than the preceding. This process naturally 

 finds its full illustration in forms with reticulate steles. The basal 

 protostele of the specialized branch is therefore a coenogenetic feature, 

 not strictly primitive. The departure from dichotomy was entered 

 upon at a very early period in the history of the Filicales. 



Viewing the group of Filicales as a whole it may be said that the 

 branches do not hold any regular position with respect to the leaves, 

 and where the branches do arise in some relation to leaves, this associa- 

 tion is, in its evolutionary origin, a secondary phenomenon attributable 

 to possible biological advantages, one of which may be the protection of 

 the young bud. In such cases the stele of the axillary branch may 

 arise independently of the subtending leaf-trace, or may sometimes be 

 confluent with it at base. As to adventitious buds on the fronds and 

 the formation of new individuals from them, this is the most recently 

 evolved method of branching ; but it is difficult to relate it to the usual 

 method, in which the branches always spring from buds laid down at 

 the growing apex of the mother-axis. 



Cone of Selaginella pallidissima.* — S. L. Ghose describes the 

 external morphology of the cone of Selaginella })aUidisswia. It is a 

 branched structure up to 5 cm. long. The sporophylls are very little 

 differentiated from the ordinary vegetative leaves and are inserted quite 

 loosely on the axis, so that the cone does not at all form a separate com- 

 pact structure. The sporophylls of the upper plane are quite sterile,. 

 and only those of the lower plane have sporangia, one in the axil of each 

 sporophyll. Megasporangia and microsporangia are distributed indis- 

 criminately on the cone. Sometimes megaspores are unequal in size. 

 Microsporangia are saddle-shaped. The cone can be regarded as a very 

 primitive one on account of its big size, branched nature, loose insertion 

 of sporophylls, little differentiation of the latter from ordinary foliage 

 leaves, and indiscriminate distribution of megasporangia and micro- 

 sporangia on the axis. The absence of any dorsal flap or ridge on the 

 comparatively simple sporophylls of S. pallidissima and a comparison of 

 the more comple:^ sporophylls of S. spinosa, S. Emmeliana, S. serpens^ 



* Journ, Bombay Nat. Hist. Soc, xxv. (1917) pp. 284-9 (1 pi.). 



