ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 639 



theories relative to its different constituents. 2. A discussion of the 

 theories held at the beginning of the twentieth century, relating to the 

 evolution of the different parts of the vascular tissues and special sections 

 dealing with the stele, bundles, etc. 3. New observations as to the 

 ontogenic development of vascular tissues in the Cryptogams, Gymno- 

 sperms, Dicotyledons, and Monocotyledons. The following are the chief 

 conclusions based upon the results obtained : — In the course of evolution 

 the vascular tissues show successive phases, each characterized by a 

 special arrangement of vessels and sieve-tubes. All the phases can be 

 grouped under two cycles, the first of which starts with the centric phase 

 and ends with the opposite phase ; the second cycle starts with the 

 alternate phase and ends with the superposed or peripheral phase. Each 

 phase must be regarded as a successive stage of the same general type. 

 Root-structure differs from stem-structure not because of difference in 

 organ, but because the two structures correspond to two different phases 

 of evolution. Ancestral characters are better preserved in the root than 

 in the stem. In the majority of Cryptogams the stem only reaches the 

 first cycle of evolution ; in fossil plants the stem and branches have 

 reached the first phases of the second cycle, while in recent plants there 

 has been a more or less rapid advance in evolution. Many living 

 Phanerogams show the first phases at the base of the stem, but these are 

 suppressed or only transitory above the hypocotyl. The root shows 

 only a slow advance, chiefly seen in the gradual absorption of primitive 

 conducting elements. Among the Ferns the stem has made relatively 

 slow advance, and hence the stem tends to resemble the root ; the reverse 

 is the case with the Palms. When the stem has just reached the alternate 

 phase, we get the most perfect case of vascular continuity. The vascular 

 structure is more complicated in proportion to the development of the 

 plant ; if the groups remain separate the structure is polystelic ; if they 

 fuse it is solenostelic. Further complications arise owing to the partial 

 or complete fusion of different groups in different phases of develop- 

 ment. The author regards it as more logical to start from the leaf in 

 order to correctly interpret the organization of the stem, since the leaf 

 preserves primitive structures for a longer time, e.g. the cotyledons. 

 Failure to estimate the true course of evolution may be traced to want 

 of due regard to the vascular structure of the leaf. 



Leaf-symmetry and Exotrophy.* — K. Boshart has studied the 

 causes of variation in leaf-form in Phanerogams, Ferns, Lycopods, and 

 Mosses. The author finds that while the relative size of the different 

 parts of the leaf is dependent upon the area assigned to them at the 

 growing-point, the subsequent development is proportional to the 

 amount of food-material supplied to them. Another important factor 

 is the direction of the flow of food-material, a larger supply on one side 

 causing asymmetry in the leaf-form. Neither gravity nor light appear 

 to cause any modification of leaf-form. In pinnate leaves the form 

 corresponds to the system of venation, and in dorsiventral forms 

 asymmetry and anisophylly are expressions of general symmetry. In 

 plants like Goldfussia and Klugia the anisophylly is due to the dorsi- 



* Flora, ciii. (1911) pp. 91-124 (14 figs.). 



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